Onshore–offshore trends in community structural attributes: death assemblages from the shallow continental shelf of Texas

Continental Shelf Research 19 (1999) 717 — 756

Onshore—offshore trends in community structural attributes: death assemblages from the shallow continental shelf of Texas George M. Staff *, Eric N. Powell
Austin Community College, NRG Campus, Geology Department, 11928 Stonehollow Drive, Austin, TX 78758 USA
Haskin Shellfish Research Laboratory, Rutgers University, 6959 Miller Ave., Port Norris, NJ 08349, USA Received 24 February 1998; accepted 18 May 1998

Abstract Death assemblages were compared at three sites on the inner continental shelf of Texas using the community attributes of taxon richness, taxonomic composition, habitat tiers, and feeding guilds, by means of three descriptor variables, numerical abundance, paleoproduction (biomass-at-death), and paleoingestion (lifetime ingestion, a measure of energy flow). Each death assemblage was compared to the co-occurring life assemblage and to six other death assemblages covering a transect from the estuary to the continental slope. Analysis of death assemblage composition is increased by at least a factor of two if fragments are included. In each of these nine assemblages on the bay-to-slope transect, the whole-shell component of the assemblage was adequate for a thorough analysis of community guild, tier, and taxon structure. The assemblage types were each unique in a combination of key abundance, paleoproduction and paleoingestion-derived community attributes. At least as important, however, were the resemblances between certain assemblages. All shelf and heterotrophic slope assemblages were characterized by predator dominance of paleoingestion. Deposit feeders and chemoautotrophs increased in importance numerically offshore, but not when evaluated by energy flow. All offshore assemblages were characterized by 40% or more of the individuals being infaunal. On the whole, tier structure was more variable than guild structure within habitat. On the whole, paleoingestion was more variable than numerical abundance or paleoproduction within habitat, probably because of the reliance of paleoingestion on long-lived taxa that are normally relatively rare. Greatest similarity was seen in paleoguild structure (a simplified guild structure) within and between habitat. Using paleoguild structure significantly limited the discrimination of assemblage types. The relative proportion of predators in the shelf and slope heterotrophic assemblages was striking. Comparing predator lifetime ingestion with prey lifetime production reveals that the slope cold-seep assemblages and bay assemblages have a large surplus of primary consumers,

* Corresponding author. Tel.: 001 512 388 3682; e-mail: [email protected]. 0278—4343/99/$ — See front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 02 7 8— 4 34 3 ( 97 ) 0 01 0 8— 3

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whereas the normal slope assemblage and the three inner shelf assemblages are overrepresented by predators. Assuming that the proportions of predator and prey indicate the relative importance of non-preservable prey, non-preservable prey were relatively more abundant on the shelf and slope, and, in fact, nonpreservable species contributed about 90% of the individuals to the life assemblage at the three inner shelf sites.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction In spite of the efforts of a great many paleontologists, it is still a difficult and uncertain task to reconstruct even the preservable portion of paleocommunities from their fossil assemblages. Difficulties arise from two sources; the degradation or loss of data from the original assemblage/community and the limited knowledge of modern benthic environments and the autecologic and taphonomic processes at work in them. Increasingly, efforts have been directed at examining the taphonomic processes in modern environments in order to determine the degree and kind of alteration of the preservable fauna that can be expected in various habitats (Brett and Baird, 1986; Staff et al., 1986; Miller, 1988; Meldahl and Flessa, 1990; Russell, 1991; Callender and Powell, 1992; Powell et al., 1992; Bartley, 1996). These studies not withstanding, relatively few modern benthic environments and their assemblages have been described quantitatively with regard to the taphonomic milieu and the impact of those processes on death assemblage formation (e.g., Aller, 1995; Greenstein and Moffat, 1996; Springer and Flessa, 1996; Martin et al., 1996; Powell et al., 1989) and even fewer have attempted to relate taphonomic process and community structure within environmental gradients to obtain a more holistic view of assemblage formation (e.g., Callender and Powell, 1992, 1997). Conceptual models, however, provide the underpinnings for such an approach (Speyer and Brett, 1988; Brandt, 1989; Meldahl and Flessa, 1990; Aberhan and Fu¨rsich, 1991) In this paper, we examine the community structure of the preservable component of the living community and its death assemblage at three nearby stations on the shallow continental shelf of Texas. A previous study described the taphonomic condition of these death assemblages (Staff and Powell, 1990a) and observed distinct taphonomic signatures related to sediment type and water depth. An equivalent paleoenvironment might produce an outcrop that could easily include all three stations within a single broad environment of deposition and describe the fauna as a single fossil association. In this contribution, we examine the relationship of community attributes along the same small-scale environmental gradients; and ask specifically, what is the small-scale variability in community characteristics of these death assemblages and are some community attributes more reliable than others for reconstructing the preservable component of the original community from the death assemblage? Finally, we compare the attributes of this shelf assemblage with nearby estuarine and continental slope assemblages to compare variability at the alpha (within-habitat) and beta (between-habitat) levels of ecosystem structure (e.g., Harrison et al., 1992).

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2. Methods 2.1. Field collection Three sites (stations 1, 2, and 3) were located in water depths of 15, 19, and 22 m, at distances of 5 km (27° 32.55N, 97° 10.23W), 7 km (27° 32.58N, 97° 9.75W), and 9 km (27° 32.97N, 97° 8.61W), respectively, offshore of Corpus Christi Bay, Texas, on the inner continental shelf (Fig. 1 in Staff and Powell, 1990a). All three locations are below normal wave base but above storm wave base. Substrate was composed predominately of sand and silt with varying amounts of gravel and clay. All three stations were south of the coastal area influenced by the Mississippi River plume (e.g., Cochrane and Kelly, 1986; Walker and Rouse, 1993; Sahl et al., 1997); thus, salinities and oxygen contents remained at levels typical of open continental shelves. Curray (1960) and Brown et al. (1976) describe the geology of the region. Staff and Powell (1990a, b) and Callender et al. (1992) discuss the taphonomic milieu. Shelf fauna were collected using a 30-cm square, 1-m deep box core. Each station was sampled about every 6 weeks over a 14-month period during 1986/1987. Normally, four box cores were taken per station per sampling occasion. The entire upper 15 cm of each box core sample was sieved onboard ship through a 1-mm mesh sieve immediately after collection. Each sample, therefore, represented four 0.09 m box cores each representing about 0.01 m. 2.2. Data analysis Samples were stained with Rose Bengal to facilitate sorting of live individuals. Individuals were sorted and identified to species. Taxonomic authorities included Abbott (1974), Defenbaugh (1976), Andrews (1977), Uebelacker and Johnson (1984), and Williams (1984). All whole specimens were tallied. Molluscan fragments were tallied if beaks (for bivalves) and apexes (for gastropods) were present. The maximum anterior-posterior length for each bivalve and the apex-to-adapical tip length for each gastropod was measured to the nearest 0.1 mm. For bivalves, each disarticulated valve was counted. In environments where taphonomic loss rates or biological predation rates are high, little chance exists for both valves of a disarticulated bivalve to survive (Powell and Stanton, 1996; Callender and Powell, 1997). Powell (1992) suggested that rates of taphonomic loss might be lower on the continental shelf. Evidence from taphonomic signatures of this shelf assemblage do not discredit this surmise (Staff and Powell, 1990a,b). Accordingly, the contribution of bivalves to community attributes might be somewhat overestimated; in the extreme by a factor of two. Larger species and individuals will likely be the most biased as the likelihood of their preservation is highest (Powell et al., 1989; Callender and Powell, 1997). 2.3. Paleoenergetics We examined the assemblages using the basic community attributes of species composition and abundance of the constituent species, and also using two attributes

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indicative of community energetics, paleoingestion and paleoproduction. Living communities can be defined in terms of the abundance and biomass of the constituent species and the contribution of each species to the community’s energy budget. Paleoproduction and paleoingestion are the paleoecological analogues of biomass and energy flow through the consumer food chain in living communities (Powell and Stanton, 1995). We use the prefix ‘‘paleo’’ to distinguish time-averaged attributes from ecological attributes. Because the individuals in a fossil or death assemblage have completed their life spans, measures of community energetics must be integrated over animal life spans (Staff et al., 1986; Powell et al., 1989). Paleoproduction is the net production of somatic tissue over the animal’s life span rather than the standing crop at any one time. Paleoingestion is the estimated minimal amount of energy required to have sustained the preserved individual over its life span rather than the amount of energy processed during a discrete time interval as generally used in ecological studies. For heterotrophs, paleoingestion is equivalent to the amount of food ingested. Paleoingestion, as described by Powell and Stanton (1995), is calculated by: P #PPJR#RJR A IJR" JR" EJR a a

(1)

where AJR is the energy assimilated (in joules) over the individual’s life span, PEJR, paleoproduction, is the portion of net production devoted to somatic growth over the individual’s life span, PPJR is the portion of net production devoted to reproduction over the individual’s life span, RJR is the amount of energy respired over the individual’s life span, and IJR, paleoingestion, is the amount of energy consumed (in joules) over the animal’s life span or the assimilated energy over the assimilation efficiency, a. Methods of calculation of the terms in Eq. (1) are presented in Powell and Stanton (1995). Of necessity, only whole individuals from the death assemblage were used, there being no way to measure the original size of fragmented specimens; and only mollusks from the life assemblage were used because the Powell and Stanton (1985) model addressed only molluscan taxa. 2.4. Guild and tier structure The use of guild and tier structure to describe community and assemblage structure has a long history in ecology and paleoecology (e.g., Scott, 1978; Fauchald and Jumars, 1979; Ausich and Bottjer, 1982; Dauer, 1984). Simberloff and Dayan (1991) review the history and theory of application of this approach. We used two types of guild structure, one based on trophic information for modern organisms and the other, a simplified paleoguild structure more likely to be recognized in the fossil assemblage. The more complicated set comprised six trophic guilds: grazers/herbivores, chemoautotrophic species (for brevity, we use this term when referring to species with symbiotic chemoautotrophic bacteria), low-level filter feeders, high-level filter feeders, predators/parasites, and deposit feeders. Authorities, besides those previously referenced for taxonomy, included Gosner (1971), Fauchald and Jumars (1979), Stanton et al. (1981), Riedl (1983), and Staff et al. (1985). The simplified set

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comprised four paleoguilds: high-level filter feeders, low-level filter feeders, predators/parasites, and deposit feeders/grazers. In simplifying, we assigned the chemoautotrophic lucinids to the low-level filter-feeder guild. The presence of symbionts is not easy to detect in fossils (Jones et al., 1988; Seilacher, 1990) and the lucinacean taxa would certainly be considered filter feeders if symbiosis went undetected (Allen, 1958; Liljedahl, 1992). We combined the grazers/herbivores with the deposit-feeders because both probably feed on bacteria, algae, and infusoria (e.g., Kofoed, 1975; Bianchi and Levinton, 1981). Recognizing that tellinaceans may be either filter feeders or deposit feeders (Hughes, 1969; Reid and Reid, 1969; Trevallion, 1971), but having little information for the Texas species, we assigned all of them to the deposit-feeder guild. We distinguished four habitat tiers: infauna, semi-infauna, low-level epifauna, and high-level epifauna. Semi-infauna are those species whose body mass straddles the sediment—water interface, such as Mercenaria mercenaria, Atrina serrata, and Polinices duplicatus. Low-level epifauna include species whose bodies rest on or just off the sediment surface and who feed just above the sediment—water interface. Mussels are good examples. High-level epifauna comprise species that can raise their feeding structures significantly above the sediment surface either by being erect or attached to other taxa. Mobile forms that typically climb on top of other species and most vagrant benthos were also included amongst the high-level epifauna. 2.5. Human intervention Human activites affect nearly all present-day assemblages in some way (Frey et al., 1987; Aitken et al., 1988; Aronson, 1990; Walker, 1995), and these effects potentially compromise comparisons between the life and death assemblages and the application of findings to the fossil record. Fishing activities are particularly significant in their effect on the present-day structure of benthic communities (e.g. Ismail, 1995; Cade´e et al., 1995; Beukema and Cade´e, 1996) and particularly pervasive on the continental shelf (Rumohr and Krost, 1991; Witbaard and Klein, 1994; Hall, 1994; Morton, 1996). The Texas continental shelf is particularly impacted by the shrimp fishery, a bottom trawl fishery (Larson et al., 1989; Flint and Rabalais, 1991) and by the erection of oil and gas production platforms (Fucik and El-Sayed, 1979; Kennicutt et al., 1996; Peterson et al., 1996). Nearshore hypoxia, a persistent problem off Louisiana and North Eastern Texas, does not extend southward into this area (Rabalais et al., 1993, 1995). Shrimping can be expected to negatively impact the larger and longer-lived species, particularly shallow infauna and semi-infauna (see, for example, Rumohr and Krost, 1991; Eleftheriou and Robertson, 1992). Shrimping activities are normally less intense near oil and gas production platforms, which were within several kilometers of our shallowest station (1). Thus, the deeper stations, (2) and (3), were likely more impacted by this activity. The frequency of impact is uncertain, we never observed shrimping on our sites, however shrimping likely impacts much of the inner Texas shelf at least once per year. Given the sedimentation rate on the Texas shelf (Siringan and Anderson, 1994) and the depth of the box core samples, it is unlikely that shrimping activities have significantly impacted the community attributes interred

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over many years in the death assemblage. Comparison to the present-day life assemblage, however, must be made cautiously.

3. Results 3.1. Taxonomic composition The numerically dominant taxa and those dominating paleoproduction and paleoingestion (those making up approximately 1% or more of the assemblages) at each of the three stations are shown in rank order in Tables 1—3. Few of the living taxa accounting numerically for 1% or more of the assemblage are preservable, regardless of station (14.9, 4.0, 8.5, respectively). At only one station, and for only one species, does a preservable taxon account for more than 5% of the life assemblage. The paucity of important preservable species at these three stations is not unusual for the Texas shelf (Harper et al., 1981; our personal observations). Regardless of the basis for comparison, most of the dominant, potentially preservable taxa in the living community are represented in the death assemblage. In most previously studied cases, preservable taxa are faithfully preserved in death assemblages and these sites are no exception (Staff et al., 1986; Powell et al., 1989; Kidwell and Flessa, 1995; but see Callender and Powell, 1997 for an exception on the nearby Texas continental slope). However, the relative importance, as indicated by the percentage of the entire assemblage made up by each taxon, changes greatly for most taxa when the living community and death assemblage are compared, even though the living community was sampled frequently over more than one year (live-dead comparisons typically improve with longer time-span sampling — Staff and Powell, 1988; Valentine, 1989; Kidwell and Flessa, 1995). The number of taxa collected in the death assemblage is always at least double the number of preservable taxa collected (Table 4). That time averaging and differential preservation of taxa alter the relative importance of the individual taxa and increase taxon richness, as observed in other nearby environments (Staff et al., 1986), is not unexpected. However, many dominant death assemblage species were not routinely collected alive. If the top 10 taxa are compared, in two of three cases, fewer than half of the top 10 preservable taxa in the life assemblage are present in the top 10 taxa in the death assemblage (Table 5). If the rank orders of taxa given in Tables 1—3 are compared by Spearman’s rank correlation, the rank orders are found to be significantly correlated in only one case, the shallowest station (Table 6). If the preservable life assemblage is compared just with the whole shell component of the death assemblage, the comparability in top 10 taxa is somewhat improved, as is the rank order agreement. Thus, the preservable life assemblage and the death assemblage agreed poorly in their dominant taxa and in the rank-order abundance of those taxa. A substantial fraction of this assemblage was present as fragments, typically about 50% (Tables 1—3). Most of the top 10 taxa in the death assemblage were in the top 10 taxa for the fraction of the assemblage present as whole individuals (Table 5). The rank-order abundances were significantly correlated in all three cases (Table 6). Thus,

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at this coarse level, focusing in on the whole shell component did not markedly change the structure of the assemblage. Paleoproduction and paleoingestion, perforce, require whole individuals. The rank orders for whole individuals based on abundance and paleoproduction were correlated in each case, with half or more of the taxa in the top 10 by paleoproduction Table 1 Rank-order percentage of the dominant taxa for the life and death assemblages at the shallowest station (Station 1) ranked by numerical abundance, paleoproduction, and paleoingestion Numerical Abundance Life Assemblage

Death Assemblage

All Taxa

%

Preservable Taxa

%

All Individuals

Magelona sp. Aglaophamus verrilli Abra aequalis Paraprionospio pinnata Mediomastus californiensis ¹haryx sp.

27.4 14.8

Abra aequalis ¹ellina texana

66.3 10.9

Abra aequalis Natica pusilla

10.2 10.2

Nucula proxima Echinoid plate

4.9 4.8

Urchin spine Echonoid plate

8.1 6.1

¸inga amiantus Anachis avara

7.9 5.2

10.0

Urchin spine

4.8

5.0

Natica pusilla

2.9

Sipunculida sp. ¸umbrineris verrilli Nemertina sp.

3.1 2.7

¸inga amiantus Corbula suifitana

1.1 1.0

Anachis avara ¹ellina texana

3.5 3.3

Nasssarius acutus Anadara transversa ¹ellina texana Parvilucina sp.

4.8

3.1

Nasssarius acutus ¸inga amiantus

2.3 2.1 1.7

¹erebellides stroemi Armanda agilis

1.5

Anadara transversa Mulinia lateralis Mercenaria campechiensis Parvilucina sp.

2.5

Decapoda sp. B ¹ellina texana

Anadara transversa Chemnitzia sp.

1.1

Nucula proxima

1.0

Processa hemphilili Amphipholis squamata

1.0

Total Taxa 17,196 Individuals [85.9%]

(1 (1

Alabina cerithidioides Cylichnella bidentata ¹rachycardium muricatum Corbula contracta Crab fragment Nucula proxima

1.0

90

Total Taxa 41 2,653 Individuals [97.7%]

Total Taxa 186,580 Individuals [79.8%]

% 23.9 20.4

4.9

2.1 1.9 1.3 1.2 1.0 1.0 1.0 1.0 1.0

138

Wholes Only Natica pusilla Abra aequalis

Mercenaria campechiensis Mulinia lateralis Alabina cerithidioides Corbula contracta Cylichnella bidentata Nucula proxima Acteocina canaliculata Epitonium apiculatum Anadara ovalis ¹rachycardium muricatum Corbula swiftiana Nannodiella vespuciana Odostomia teres Chione clenchi ¹urbonilla sp. A Calotrophon osteracum Dentalium texasianum Total Taxa 85, 814 Individuals [76.4%]

% 30.2 13.3

4.2 3.8 2.9 2.1 2.0 1.9 1.6 1.2 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 121

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Table 1 (Continued) Paleoproduction

Paleoingestion

Preservable % Life Assemblage

Death Assemblage

Natica pusilla Abra aequalis

30.9 30.3

Natica pusilla ¹ellina alternata

Noetia ponderosa ¹ellina texana

12.2

¹ellina texana

8.6

Abra aequalis

5.1

10.4

Abra aequalis

8.0

¹urbonilla sp. E

4.5

Anadara ovalis Anadara transversa Anachis avara

4.6 3.1

Natica pusilla Noetia ponderosa

4.5 1.1

2.9

¹ellina texana

1.1

Raeta plicatella Crepidula fornicata Nassarius acutus Polinices duplicatus Nuculana concentrica ¸inga amiantus Oliva sayana Mulinia lateralis Chione clenchi Calotrophon osteracum Macoma tenta ¹ellina aequistriata Kurtziella limonitella ¹urbonilla sp. A Mercenaria campechiensis Nannodiella vespuciana Corbula contracta Cylichnella bidentata Dentalium texasianum Crepidula convexa

2.5 2.4

¹urbonilla sp. H (1 Atrina serrata (1

2.4 2.3

Nucula proxima (1

Atrina serrata 3.0 Nassarius acutus 2.4 Nucula proxima

2.3

¸inga amiantus 1.8 ¹erebra protexta 1.4 ¹urbonilla sp. H 1.0 ¹ellina alternata 1.0 ¹urbonilla sp. E

1.0

Parvilucina sp.

1.0

Total Taxa 0.38 gram [95.7%]

32

Total Taxa 73.9 gram [72.4%]

% 19.7 18.2

Preservable % Life Assemblage

Dealth Assemblage

Nassarius acutus ¹erebra protexta

Natica pusilla 51.3 Kurtziella 13.9 limonitella Nassarius acutus 11.6

74.8 7.1

1.6

Nannodiella vespuciana ¹ellina alternata Macome tenta

%

7.7 3.2 1.5

Nuculana 1.5 concentrica ¹urbonilla sp. A 1.4 ¹riphora perversa 1.0 ¹ellina texana ¹urbonilla sp. H

1.0 1.0

Abra aequalis

1.0

1.6 1.6 1.5 1.4 1.3 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 114

Total Taxa 152.1 kJ [98.7%]

30

Total Taxa 48,474.1 kJ [93.9%]

112

Note: The percentage of the assemblage represented by the ten most abundant taxa is shown in brackets below the assemblage totals for taxa and individuals (grams, kilojoules) collected. Life and death assemblage paleoproduction and paleoingestion were based on only the molluscan and whole-shell components, respectively. Death assemblage paleoproduction and paleoingestion represent lifetime totals; preservable life assemblage data represent the totals up to the time of collection. Fewer taxa used for paleoproduction and paleoingestion is necessitated by the absence of adequate length-to-biomass regressions and size-to-age relationships for some rare species. Values represent a total collection of eight 6-week samplings totaling 2.88 m (0.44 m).

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also in the top 10 by abundance (Tables 5 and 6). Agreement between abundance and paleoingestion was much poorer; only one station showed a significant correlation in rank order. Larger taxa, that disproportionately affect paleoproduction, tended also to be numerically abundant. Longer-lived taxa did not. These longer-lived taxa, that disproportionately affect paleoingestion, were disproportionately represented by predators which typically are rarer species. If the dominant 10 taxa in the life and death assemblages are compared based on paleoproduction and paleoingestion, the two assemblages match no better than they did based on numerical abundance. That is, the taxon composition of the life assemblage over this sampling period agreed relatively poorly with the death

Table 2 Rank-order percentage of the dominant taxa for the life and death assemblages at the middle station (Station 2) ranked by numerical abundance, paleoproduction, and paleoingestion Numerical Abundance Life Assemblage All Taxa

%

Magelona sp. 35.3 Mediomastus 11.9 californiensis ¹haryx sp. 9.8 Aglaophamus 8.7 verrilli Paraprionospio 8.7 pinnata ¸umbrineris verrilli 3.3 Nemertina sp. 3.1 Prionospio cristata 2.1 Sipunculida sp. 1.9

Numerical Abundance Death Assemblage Preservable Taxa

%

All Individuals

¹ellina texana Abra aequalis

29.0 22.0

Abraa equalis Natica pusilla

Corbula swiftiana 10.8 Mercenaria 4.6 campechiensis Alabina 4.6 cerithidioides Solen viridis 4.3 ¸inga amiantus 3.1

Urchin spine ¹ellina texana

8.2 ¸inga amiantus 4.4 3.4 Anadara transversa 4.2

Nassarius acutus

3.4 ¹ellina texana

3.6

¸inga amiantus Echinoid plate

3.3 2.6

Anadara transversa Mercenaria campechiensis Mulinia lateralis Anachis avara Corbula contracta

2.9 Nassarius acutus 2.5 Mercenaria campechiensis 2.5 Corbula contracta 2.2 Anachis avara

¹ellina texana Abra aequlis Scoloplos rubra

1.7 1.3 1.1

Anachis avara Parvilucina multilineata Nucula proxima Natica pusilla Nuculana acuta

Cirriformia sp. A

1.0

Anadara transversa 1.2

Nuculana acuta

Diopatra cuprea

1.0

Corbula contracta

1.0

¹erebellides stroemi 1.0

Nassarius acutus

1.0

Decapoda sp. B Prionospio pygmaea Corbula swiftiana

1.0 1.0

Parvilucina sp. ¹ellina versicolor

1.0 1.0

¹rachycardium muricatum Alabina cerithidioides Nucula proxima Parvilucina sp.

1.0

Crab fragment

1.0

Ophiopholis aculineata Ophiothrix angulata

1.0

¹ellina alternata

1.0

Total Taxa 12,554 Individuals [88.5%]

2.6 2.6 2.4 1.7 1.6

%

Wholes Only

48.5 Abra aequalis 10.7 Natica pusilla

1.6 Mulinia lateralis 1.3 Nuculana acuta 1.1 ¹rachycardium muricatum 1.1 Alabina cerithidioides 1.0 Cylichnella bidentata 1.0 Parvilucina sp. 1.0 Nucula proxima 1.0 Acteocina canaliculata Polinices duplicatus

% 48.7 13.2

2.2 1.5 1.4 1.3 1.2 1.0 1.0 1.0 1.0 1.0 1.0

1.0 96

Total Taxa 744 Individuals [85.7%]

50

Total Taxa 106 82,182 Individuals

Total Taxa 91 40,096 Individuals

[85.9%]

[85.1%]

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Table 2 (Continued) Paleoproduction Preservable Life Assemblage

Paleoingestion Death Assemblage

%

Preservable Life Assemblage

Abra aequalis 23.6 ¹ellina texana 20.7 Noetia ponderosa 14.1

Abra aequalis ¹ellina texana ¹ellina alternata

33.0 13.5 9.0

Balcis jamaicensis 32.7 Abra aequalis 23.8 Corbula swiftiana 11.1

Sinum 12.9 perspectivuum Oliva sayana 9.2 Corbula swiftiana 3.9 Macoma tenta 1.7 Parvilucina 1.6 multilineata ¹ellina alternata 1.6

Nassarius acutus

5.9

¹ellina texana

Raeta plicatella Natica pusilla Anadara ovalis Anadara transversa

5.9 5.0 4.7 2.4

Mulinia lateralis

2.1

Anachis avara ¹ellina versicolor Nassarius acutus ¸inga amiantus

1.4 1.3 1.2 1.0

¹ellidora cristata Anachis avara ¸inga amiantus Oliva sayana

1.7 1.7 1.6 1.0

Balcis jamaicensis

1.0

1.0

Nucula proxima Natica pusilla Alabina cerithidioides ¹erebra arcas

1.0 1.0 1.0

Dentalium texasianum Corbula contracta Polinices duplicatus Nuculana concentrica Cylichnella bidentata Dinocardium robustum Nannodiella vespuciana

Total Taxa 0.74 gram [90.7%]

%

1.0

42

Total Taxa 25.8 gram [83.2%]

1.0 1.0 1.0

%

Death Assemblage

% 53.2 11.3 11.3

7.3

Nassarius acutus Abra aequalis Nannodiella vespuciana Natica pusilla

Sinum perspectivum Noetia ponderosa Nassarius acutus Oliva sayana

3.9 3.4 3.3 2.9

¹ellina alternata ¹ellina texana Raeta plicatella Cadulus sp.

3.9 3.4 1.3 1.2

Nannodiella vespuciana Nucula proxima ¸ittoridin barretti ¹ellina versicolor Parvilucina multilineata Alabina cerithidioides ¹erebra arcas

2.5

¹ellidora cristata

1.0

2.5 1.1 1.0 1.0

Nucul proxima Anadara ovalis

1.0 1.0

7.1

1.0 1.0

1.0 1.0 1.0 88

Total Taxa 127.0 kJ [93.4%]

40

Total Taxa 7,889.8 kJ [94.7%]

86

Note: The percentage of the assemblage represented by the ten most abundant taxa is shown in brackets below the assemblage totals for taxa and individuals (grams, kilojoules) collected. Life and death assemblage paleoproduction and paleoingestion were based on only the molluscan and whole-shell components, respectively. Death assemblage paleoproduction and paleoingestion represent lifetime totals; preservable life assemblage data represent the totals up to the time of collection. Fewer taxa used for paleoproduction and paleoingestion is necessitated by the absence of adequate length-to-biomass regressions and size-to-age relationships for some rare species. Values represent a total collection of seven 6-week samplings totaling 2.52 m (0.36 m).

727

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

assemblage by rank-order abundance, rank-order paleoproduction, or rank-order paleoingestion. To this extent, time averaging has more or less equivalently affected the death assemblage by each of these three measures.

Table 3 Rank-order percentage of the dominant taxa for the life and death assemblages at the deepest station (Station 3) ranked by numerical abundance, paleoproduction, and paleoingestion Numerical Abundance Life Assemblage All Taxa

%

Magelona sp. 29.2 Mediomastus 12.5 californiensis Aglaophamus 11.9 verrilli Paraprionospio 10.0 pinnata Abra aequalis 4.4 ¹haryx sp. 4.1 ¸umbrineris verrilli 4.1 Prionospio cristata 3.3

Numerical Abundance Death Assemblage Preservable Taxa %

All Individuals

%

Wholes Only

%

Abra aequalis Corbula swiftiana ¹ellina texana

46.3 11.0

Abra aequalis Natica pusilla

57.7 7.9

Abra aequalis Natica pusilla

53.5 9.8

10.2

¹ellina texana

6.7

¹ellina texana

8.3

Nucula proxima

10.0

Urchin spine

6.2

¸inga amiantus

3.4

Urchin spine Natica pusilla Mercenaria campechiensis Amaea mitchelli

6.3 2.8 2.3

Echinoid plate Nassarius acutus ¸inga amiantus

2.4 2.3 2.0

Nassarius acutus 3.3 Anadara transversa 2.6 Anachis avara 1.7

1.1

1.4

Nuculana acuta

1.6

1.1

1.3

1.0

Speocarcinus lobatus Nemertina sp. Decapoda sp. B

3.0

¹erebra dislocata

1.0

Anadara transversa Nuculana acuta

2.6 1.5

¹erebra arcas ¸inga amiantus

1.0 1.0

Anachis avara Nucula proxima

1.0 1.0

Sipunculida sp.

1.2

Nuculana acuta

1.0

1.0

Corbula swiftiana

1.1

Corbula contracta

1.0

Mercenaria campechiensis Alabina certhidioides

Mercenaria campechiensis Nucula proxima Alabina cerithidioides Parvilucina sp.

1.0

Odostomia teres

1.0

¹ellina texana Nucula proxima

1.0 1.0

Echinoid plate

1.0

1.0 1.0

Cirriformia sp. A

1.0

Caprellida sp.

1.0

Ceratocephale oculata Notomastus sp. Diopatra cuprea

1.0

Corbula contracta Mercenaria mercenaria Nuculana concentrica Dentalium sp. ¹rachycardium muricatum ¹erebra texana

1.1 1.0

1.0 1.0 1.0 1.0

1.0 1.0

Total Taxa 98 18,489 Individuals [95.1%]

Total Taxa 52 1,772 Individuals [92.0%]

Total Taxa 13 68,707 Individuals [88.7%]

Total Taxa 86 33,839 Individuals [86.6%]

728

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

Table 3 (Continued) Paleoproduction Preservable Life Assemblage

Paleoingestion %

Noetia ponderosa 24.7 ¹ellina alternata 24.7 ¹ellina texana 15.6 Abra aequalis 9.6 Solen viridis 8.9 Nucula proxima 2.8

Death Assemblage %

Preservable Life Assemblage

Abra aequalis 45.9 ¹ellina texana 29.9 Natica pusilla 3.2 Nassarius acutus 2.5 ¹ellina alternata 2.3 Nuculana 1.2 concentrica Dentalium sp. 1.2 Anachis avara 1.1 Noetia ponderosa 1.0 Anadara brasiliana 1.0

¹urbonilla sp. G 57.0 Abra aequalis 8.1 ¹ellina alternata 7.7 Noetia ponderosa 6.2 ¹ellina texana 5.4 Solen viridis 3.7

Abra aequalis ¹ellina texana Nassarius acutus Dentalium sp. ¹urbonilla sp. A ¹urbonilla sp. C

33.1 21.8 13.3 5.6 3.5 2.8

Nucula proxima ¹urbonilla sp. A ¸ima sp. ¹urbonilla sp. F

2.3 2.2 2.1 1.0

2.5 2.3 1.8 1.4

Solariorbis infracarinata Anatina anatina

1.0

Nucula proxima ¹erebra arcas Natica pusilla Nuculana concentrica ¹ellina alternata

1.1

Corbula swiftiana Acteon punctostriatus

1.0 1.0

Nannodiella vespuciana Odostomia teres Nuculana acuta

1.0 1.0

Anachis avara Noetia ponderosa Solen virids

1.0 1.0 1.0

Anatina anatina Corbula swiftiana ¹urbonilla sp. G Macoma tenta

2.6 2.4 1.5 1.1

¸inga amiantus

1.0

Natica pusilla

1.0

Mercenaria campechiensis Solen viridis

¸ima sp. ¹erebra texana

1.0 1.0

¹erebra arcas 1.0 Crepidula fornicata 1.0 Pitar cordatus

Total Taxa 0.96 gram [93.9%]

43

Total Taxa 24.3 gram [89.3%]

1.0 1.0

%

1.0

1.0

81

Total Taxa 159.2 kJ [95.7%]

41

Death Assemblage %

Total Taxa 2,011.2 kJ [88.1%]

1.2

79

Note: The percentage of the assemblage represented by the ten most abundant taxa is shown in brackets below the assemblage totals for taxa and individuals (grams, kilojoules) collected. Life and death assemblage paleoproduction and paleoingestion were based on only the molluscan and whole-shell components, respectively. Death assemblage paleoproduction and paleoingestion represent lifetime totals; preservable life assemblage data represent the totals up to the time of collection. Fewer taxa used for paleoproduction and paleoingestion is necessitated by the absence of adequate length-to-biomass regressions and size-to-age relationships for some rare species. Values represent a total collection of nine 6-week samplings totaling 3.24 m (0.48 m).

The poor showing for paleoproduction (biomass in Staff et al., 1985), in comparison to other assemblages along the Texas coast (e.g., Staff et al., 1985, 1986; Callender and Powell, 1997), suggests that shrimping activities may have biased the life community at these stations. In effect, the representation of the larger and longer-lived individuals in the life assemblage bore little resemblance to the same for the death assemblage, and this is the result that would be anticipated from bottom fishing activities. One cannot exclude, however, the possibility that natural processes, such as recent hurricanes, may have influenced the life assemblage in a similar way. One of the largest

Total Abundance

Life Assemblage

Shallow Station 90 Middle Station 96 Deep Station 98

Location

Table 4 Taxon richness

41 50 52

Preservable Abundance

Life Assemblage

138 106 103

Total Abundance

Death Assemblage

121 91 86

Whole Shells Only

Death Assemblage

32 42 43

Life Assemblage

Paleoproduction

114 88 81

Death Assemblage

Paleoproduction

30 40 41

Life Assemblage

Paleoingestion

112 86 79

Death Assemblage

Paleoingestion

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756 729

10[89.9%]

10 [82.2%]

10[92.7%]

10 [78.4%]

10 [85.7%]

Middle station

Deep station

8 [55.6%] 80% 8 [76.6%] 80% 8 [77.6%] 80% 10 [77.6%]

10 [76.6%]

10 [55.6%]

6 [50.9%] 60% 6 [71.9%] 60% 5[84.3%] 50%

Paleoproduction

4 [50.9%] 40% 3[71.9%] 30% 4 [84.3%] 40%

Death Assemblage

10 [77.6%]

10 [76.6%]

10 [55.6%]

Preservable Whole Abundance

Death Assemblage

10 [91.7%]

10 [88.3%]

10 [98.7%]

Preservable Life Assemblage

Paleoingestion

3 [91.9%] 30% 4 [93.7%] 40% 5 [80.8%] 50%

Paleoingestion

3[91.9%) 30% 5 [93.7%] 50% 4 [80.8%] 40%

Death Assemblage

Note: Number in brackets represents the proportion of the assemblage represented by the dominant taxa. Percentage below is the fraction of taxa present in both groupings.

10 [70.4%]

Shallow station

Death Assemblage Whole Abundance

5 [55.6%] 50% 5 [76.6%] 50% 5 [77.6%] 50%

Preservable Whole Abundance

Deep station

Location

10 [88.0%]

Middle station

7 [70.4%] 70% 4 [78.4%] 40% 4 [85.7%] 40%

Death Assemblage Total Abundance

10 [75.2%]

Shallow station

Death Assemblage Preservable Whole Life Abundance Assemblage

Death Assemblage

10 [97.7%]

Location

Death Assemblage Total Abundance

Paleoproduction

Numerical Abundance

Preservable Life Assemblage

Numerical Abundance

Table 5 Number of taxa in the top 10 taxa of one assemblage(left) also present in the dominant 10 taxa of the other (right)

730 G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

Life Assemblage Numerical Abundance vs. Death Assemblage Numerical Abundance Whole Shells Only

r "0.29; Q P'0.05

r "0.42; Q P"0.04

r "0.40; Q P"0.05

Life Assemblage Numerical Abundance vs. Death Assemblage Numerical Abundance

r "0.54; Q P"0.01

r "0.16; Q P'0.05

r "0.40; Q P'0.05

Location

Shallow station

Middle station

Deep station

r "0.71; Q P"0.0005

r "0.42; Q P"0.04

r "0.73; Q P"0.0001

Death Assemblage Total Abundance vs. Death Assemblage Whole Shells Only

r "0.37; Q P'0.05

r "!0.22; Q P'0.05

r "0.35; Q P"0.04

Life Assemblage Paleoproduction vs. Death Assemblage Paleoproduction

r "0.79; Q P"0.0001

r "0.60; Q P"0.002

r "0.50; Q P"0.0025

Death Assemblage Numerical Abundance Whole Shells only vs. Death Assemblage Paleoproduction

r "0.14; Q P'0.05

r "0.06; Q P'0.05

r "0.04; Q P'0.05

Life Assemblage Paleoingestion vs. Death Assemblage

Table 6 Results of Spearman’s rank correlation analyses comparing the indicated two data types for the taxa listed in Tables1—3

r "0.54; Q P"0.01

r "0.37; Q P'0.05

r "0.15; Q P'0.05

Dealth Assemblage Numerical Abundance Whole Shells only vs. Death Assemblage

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756 731

732

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

hurricanes ever recorded in the Gulf of Mexico (Hurricane Allen) passed near this area just a few years prior to our sampling (Brooks, 1983). Moreover, larger species, more likely to be preserved in these assemblages (Powell et al., 1998), are notoriously variable over the long time spans encompassed by the 15-cm sampling depth of this study (e.g., Beukema et al., 1978; Maurer and Haydock, 1990; Schimmelmann et al., 1992; Powell et al., 1992) and, because of their rarity, are likely to have abundances less accurately estimated by the sampling program (Green and Young, 1993). Thus, the poor agreement between the life and death assemblages cannot be unambiguously ascribed to present-day fishing activities. It could be normal for the Texas inner shelf. 3.2. Taxon richness The preservable portion of the living community made up about 50% of the total taxa (46, 52, 53% for stations 1, 2, and 3, respectively), whereas they contributed less than 15% of the individuals. As anticipated from the influence of time averaging, taxon richness increased in the death assemblage relative to the preservable component of the original community at all three stations (Table 4). The two deeper stations have almost the same number of taxa in the living community and very nearly the same number in the death assemblage. The shallower station, closest to shore, has the largest number (138) of taxa in the death assemblage when compared to the number (90) in the living community and the largest number in the death assemblage overall. Storm reworking is most important at the shallow station. Staff and Powell (1990b) suggested that shells are being moved shoreward to this location from the assemblages farther offshore. The whole shell component of the death assemblage contained 88, 86, and 83% of the taxa in the entire death assemblage in stations 1, 2, and 3, respectively. Thus, few taxa were present solely as fragments and many of those, such as echinoids and crabs, are rarely present whole in any death assemblage. Thus, analysis of paleoproduction and paleoingestion used the bulk of the taxa present in the death assemblage. 3.3. Habitat tiers The life assemblage was dominated numerically by infauna and low-level epifauna, about evenly split, at all three stations (Fig. 1). The preservable life assemblage was primarily infaunal, reflecting the abundance of infaunal clams. The larger preservable animals were almost exclusively infaunal, except at the shallowest station (1) where some large semi-infauna were present (Fig. 1, paleoingestion). In contrast, paleoingestion was distributed unevenly across infauna, low-level epifauna and, at the deepest station, high-level epifauna, indicating that most epifauna, though small, were fairly long-lived. The three stations were very similar in tier structure numerically, but differed considerably by paleoproduction, and particularly by paleoingestion. Longlived epifaunal taxa, though rare numerically, and contributing relatively little to standing biomass, did contribute significantly to energy flow. Many of these taxa were epifaunal predators.

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

733

Fig. 1. Comparison of life-habit strategies between the life assemblage (left) and death assemblage (right) for each station using habitat tier categories, listed as percentages of the assemblage evaluated by total numerical abundance, preservable or whole-shell abundance, paleoproduction, and paleoingestion. Calculations of paleoproduction and paleoingestion based on the molluscan component of the life assemblage and the whole-shell component of the death assemblage only.

The whole shell component of the death assemblage had very nearly the identical tier structure of the entire death assemblage (Fig. 1), so that adding the fragmentary component did not overwhelmingly change the evaluation of assemblage tier structure. However, the few high-level epifauna that were present were principally present

734

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

as fragments, so that this tier was not well-represented by the whole shell component of the assemblage. The four tiers were more evenly represented in the death assemblage than in the life assemblage, and a trend existed in increasing representation of infauna with increasing depth. Tier structure evaluated by paleoproduction increased the fraction of the assemblage that was infaunal to some degree at each station; the trend of increasing infauna with depth remained. Paleoingestion, however, markedly changed tier structure at all but the deepest station. Semi-infauna and low-level epifauna increased proportionately in stations 1 and 2, respectively. Comparison of living community and death assemblage proportions of individuals in each habitat tier by numerical abundance reveals a shift in relative abundance from even proportions of infauna and low-level epifauna in the living community to increased dominance by infauna and semi-infauna in the death assemblage (Fig. 1). The trend towards increased infaunal representation was already present in the preservable component of the life assemblage, however, so that some portion of this shift originates in the loss of soft-bodied taxa. Infaunal proportions decrease in the death assemblage using paleoproduction when compared to paleoproduction of the preservable life assemblage, because the fraction of low-level epifauna increases. However, infauna still clearly dominant the death assemblage and the overall structure of the assemblage is not markedly changed. Tier structure using paleoingestion is distinctively different in both the life and death assemblages from that based on abundance (preservable or whole) or paleoproduction. In comparison to the preservable life assemblage, semi-infaunal organisms increase in relative importance at all stations in the death assemblage, particularly so at station 1, and significant differences exist between the life assemblage and the death assemblage at each station in the proportions of infauna and low-level epifauna present. There is also considerably more variation between stations using paleoingestion data in both the life and death assemblages. Overall, comparing the life assemblage to the death assemblage at all three stations reveals a trend of decreasing dominance of tier type in favor of an increased evenness of distribution of proportions among them in the death assemblage, regardless of whether abundance, paleoproduction or paleoingestion are used. This trend probably is due to time averaging and the differential preservation of larger individuals that tend to be more evenly distributed across tier type. Infauna are more abundant in the death assemblage in deeper water, as might be expected by the influence of the muddier substrate on community structure and preservation. Paleoingestion intensifies this trend. 3.4. Guild structure The life assemblage was numerically dominated by deposit feeders, with predators/parasites also well represented, at all three stations (Fig. 2). The preservable component was much more diverse; low-level suspension feeders increase in prominence and some grazers are present. Filter feeders and predators increase in prominence by paleoproduction, at the expense of the smaller deposit feeders. This shift is not unusual (Staff et al., 1985). Predators/parasites come to dominance using

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

735

Fig. 2. Comparison of feeding strategies between the life assemblage (left) and death assemblage (right) for each station using a complex set of guild categories, listed as percentages of the assemblage evaluated by total numerical abundance, preservable or whole-shell abundance, paleoproduction, and paleoingestion. Calculations of paleoproduction and paleoingestion based on the molluscan component of the life assemblage and the whole-shell component of the death assemblage only.

paleoingestion. The longer life span of predatory gastropods, in comparison to their prey (Powell and Cummins, 1985; Heller, 1990; Callender and Powell, 1997), as well as their relative abundance, is responsible.

736

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

Guild structure of the death assemblage is more complex, when evaluated by numerical abundance (Fig. 2). The relative importance of deposit feeders in the preservable life assemblage decreases in the death assemblage at Station 1 as the proportions of the herbivore/grazer, predator and low-level filter-feeder guilds increase. Predators are more abundant at all stations than in the preservable life assemblage. As with tier structure, removal of the fragmentary component of the death assemblage does not change guild structure markedly. Most grazers are lost, but these were uncommon anyway, and predators increase in importance proportionately. When paleoproduction data are used, the differences between the guild structures of the preservable life assemblage and whole-shell component of the death assemblage are minor (Fig. 2), with deposit feeders becoming somewhat more important in comparison to the low-level filter feeders in the death assemblage. The larger bivalves, on the average, are deposit feeders (but see the caveat on tellinids). Guild structures of the death assemblage and the preservable life assemblage are also similar, when evaluated by paleoingestion (Fig. 2). The dominant guild, predator/parasite, in the preservable life assemblage is at least as prominent in the death assemblage in two of the three stations, whereas the proportion of lowlevel filter feeders is somewhat reduced. Predators also increase in importance when the death assemblage is evaluated by paleoingestion rather than abundance or paleoproduction. Two guilds, the predators/parasites and deposit feeders, account for the majority of all individuals. The large influence of the predator guild in paleoingestion originates from the longer life spans of most neogastropods and the relatively high abundance of predators in the death assemblage at all three stations. The influence of deposit feeders originates from their relatively large size. Like habitat tiers, guild structure is similar when evaluated by abundance or paleoproduction, whether the comparison is made in the preservable life or death assemblage, whereas guild structure by paleoingestion emphasizes a different aspect of population structure in both cases. Overall, the death assemblage more closely resembles the dominance structure of the guilds in the preservable life assemblage at most stations and using most datum types (numerical abundance, paleoproduction, paleoingestion), unlike the case in tier structure where significant changes were observed within datum types and stations, for two reasons. First, certain guilds are simply unimportant, regardless of datum or assemblage type. Species with chemoautotrophic symbionts are poorly represented as is true for most shelf habitats, and particularly strikingly so for the Texas shelf (Parker, 1960; Hill et al., 1982; Calnan and Littleton, 1989; Callender and Powell, in press). Herbivores/grazers are unimportant and high-level filter feeders are nearly absent. Both attributes distinguish these assemblages from those in deeper water (Callender and Powell, 1997) and in Texas bays (Powell and Stanton, 1995). Secondly, the stations consistently are dominated by deposit feeders and predators/parasites. The tendency for infauna to increase in deeper water (Station 1 vs. Station 3) explains the similar tendency in deposit feeders. The opposite trend is present in the predators/parasites.

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

737

3.5. Paleoguild structure Guild categories can be fairly detailed because the specific feeding characteristics of the extant taxa are relatively well known. This is not always the case in the fossil record. Consequently, some categories were combined into paleoguilds to simulate the coarser categories often necessitated by the fossil record. Because herbivores/grazers and chemoautotrophs were rare in these assemblages, unlike the Texas continental slope and nearby estuaries (Powell et al., 1992, 1998), the guild structure was relatively unchanged by this transformation overall, regardless of station or datum type (Fig. 3). 3.6. Evenness Comparison of the proportions of the dominant taxa in the preservable life and death assemblages for the three stations indicates that the dominant taxa are more evenly distributed in the death assemblages. The percentage of each assemblage represented in the top ten most dominant taxa appears at the bottom of Tables 1—3. In nearly every instance, the life assemblage’s top ten dominant taxa contributed a larger portion of the assemblage than did the analogous 10 taxa in the corresponding death assemblage. Evenness has probably increased in the death assemblages due to the time averaging of temporally distinct dominant taxa in the living community. In most cases, only about half of the dominant 10 taxa in the death assemblage were also among the dominant 10 in the preservable life assemblage (Table 5), indicating that taxon dominance in the life assemblage is not a persistent feature of these communities. Paleoproduction and paleoingestion exacerbate this trend. That is, taxon dominance is even less persistent in the preservable life assemblage than when measured by numerical abundance, assuming that the death assemblage is a reasonable time integrator of life assemblage structure. The larger and longer-lived species are less persistent in their relative abundance than are the smaller species. Evenness of habitat tier and guild proportions was compared using the formula: p

p

! G ln G . . ln C

(2)

(Vandermeer, 1981) where p is the proportion represented by the ith category, P is the G total (1.0 in this case), and C is the total number of categories in the analysis. The evenness of the proportions of habitat tiers was greater for the death assemblages at all three locations for all three datum types (Table 7). Evenness changed the least for paleoingestion because infaunal dominance in the life assemblage was reduced using this measure. For guild and paleoguild structure, evenness increased in the death assemblage using numerical abundance to define guild proportions, but not so using paleoproduction or paleoingestion (Table 7). Deposit feeders and predators, respectively, became more dominant in the death assemblage when proportions were

738

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

Fig. 3. Comparison of feeding strategies between the life assemblage (left) and death assemblage (right) for each station using a simplified set of paleoguild categories, listed as percentages of the assemblage evaluated by total numerical abundance, preservable or whole-shell abundance, paleoproduction, and paleoingestion. Calculations of paleoproduction and paleoingestion based on the molluscan component of the life assemblage and the whole-shell component of the death assemblage only.

evaluated by paleoproduction or paleoingestion. Thus, guild structure was simplified using paleoproduction and paleoingestion because only a few guilds contributed the majority of the larger and longer-lived species. Smaller, shorter-lived species were more diverse in their guild structure. The opposite was true for habitat tiers.

.53

.39

.29

.27

.29

.38

.27

.42

.46

.43

Numerical Abundance Preservable Whole-shell Life Death Assemblage Assemblage

.42

.45

.20

Life Assemblage

Paleoproduction

.40

.41

.38

Death Assemblage

.33

.34

.35

Life Assemblage

Paleoingestion

.28

.27

.39

Death Assemblage

Note: Numbers represent the average evenness values for all three stations. The closer the value is to one, the more even the distribution of proportions among habitat tier or feeding guild categories.Values with decreasing evenness from the life assemblage to the death assemblage are underlined.

.49

.38

Habitat Tier Feeding Guild Paleoguild

Death Assemblage

Life Assemblage

Guild/ Tier

Numerical Abundance

Table 7 Evenness values calculated using numerical abundance, paleoproduction, and paleoingestion to compare habitat tier, guild and paleoguild proportions

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756 739

740

G.M. Staff, E.N. Powell/Continental Shelf Research 19 (1999) 717—756

4. Discussion 4.1. Fragmentation In most assemblages, a substantial fraction of the preserved remains are fragmented, and these three continental shelf sites are no exception. Identification and enumeration of fragments is a time consuming process, and, accordingly, this component is often not included in data analysis. In addition, because the size of the original animal cannot be reconstructed, fragments cannot easily be included in any trophic analysis. Potentially, then, fragmentation can bias community analysis in a number of ways. About half of all of the individuals in our collections were fragmented. However, deletion of these individuals from analysis would have had only a minor impact on most community attributes evaluated by numerical abundance. (We cannot assess the impact on energy flow, because we have not reconstructed the original size of each specimen.) Nearly all species were present as whole shells, and many of those that were not were echinoid or crab remains that are essentially never present as whole individuals. Thus species richness was not much affected (Table 4). The taxon rank orders by abundance based on all individuals were correlated at each station with the taxon rank orders evaluated from whole shells only (Tables 1—3 and 6). Eighty percent of the top ten taxa were present in the top ten when evaluated using only whole specimens at each station (Table 5). Thus, the taxonomic composition of the community was not greatly altered when only whole shells were used. Tier structure was very similar (Fig. 1). Some high-level epifauna were lost, but the proportions were not inordinately different when evaluated using only whole shells. The same could be said for guild and paleoguild structure (Figs. 2 and 3). Most grazers were lost from the analysis when whole shells were used, but grazers made up only a small proportion of the assemblage in any case. Thus, by any numerical measure, addition of the fragmented individuals, which about doubled the total data base, had only a minor impact on inferred community structure. At this site, data analysis could have been limited to whole shells without seriously compromising the study. 4.2. Differences between community attributes Community attributes of the death assemblage were evaluated by numerical abundance, paleoproduction, which emphasizes the size of the individuals, and paleoingestion, which emphasizes the size and life span of the individuals. Community structure of the death assemblage was very similar when evaluated by abundance or by paleoproduction. Rank orders of taxon abundance were significantly correlated at all three sites. Half or more of the taxa were in the top 10 taxa using abundance or paleoproduction. Tier structure and guild structure were remarkably similar. Nearly all taxa were small, and this minimized the influence of size differences when community attributes were calculated by paleoproduction. Moreover, the most common species, such as Abra aequalis and ¹ellina texana, were not notably smaller or bigger

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741

than most other species in the assemblage. For both reasons, evaluating the death assemblage by paleoproduction did not significantly change inferred community structure. Quite the opposite was true for paleoingestion. The rank orders of taxa were different in two of three cases. Tier structure resulted in the emphasis of low-level epifauna or semi-infauna, depending upon station, at the expense of the numerically dominant infauna. Guild structure emphasized the carnivore trophic level at the expense of the numerically dominant deposit feeders, but also at the expense of the less common filter feeders. The predators, mostly gastropods in this case, tend to have much longer life spans (Powell and Cummins, 1985; Heller, 1990; Callender and Powell, 1997). These predators also tend to be epifaunal or semi-infaunal (in the case of the naticids). The long-lived predators, relatively rare numerically, are, thus, principally responsible for the distinctive community structure exposed by the analysis of paleoingestion. 4.3. Predator overrepresentation Predators should always be rarer than their prey because trophic level conversion efficiencies rarely exceed 15% (Powell and Stanton, 1985). In death assemblages, preserved prey should be proportionately even more common because predators also live longer and, thus, time-averaged trophic level conversion efficiencies should be much lower. If the energy needs required by the predators, predator paleoingestion, exceeds the amount available from the preserved prey, prey paleoproduction, then predators can be said to be overrepresented in the assemblage (Powell and Stanton, 1995). [The paleoproduction/paleoingestion ratio should be distinguished from the production/biomass (P/B) ratio commonly used in ecology (e.g., Brey, 1990; Howe et al., 1988), which is not a time-averaged variable]. Predators were overrepresented at all three stations (Table 8). Preserved biomass was sufficient to cover 2, 7 and 58% (stations 1, 2, and 3, respectively) of the preserved predators’ needs. Assuming

Table 8 Predator overrepresentation, based only on whole-shell data

Location

Prey Paleoproduction

Predator Paleoingestion

Ratio

Shallow station Middle station Deep station

870.68 (1,379.56) 390.38 (5,849.24) 401.70 (692.61)

42,779.00 1,396.00 165.30

0.02 (0.03) 0.07 (0.08) 0.58 (0,65)

Note: Predators are not included in prey paleoproduction in calculating the ratio prey paleoproduction/predator paleoingestion, even though they may consume each other occasionally. Had they been, prey paleoproduction and the ratio prey paleoproduction/predator paleoingestion would have been modified as shown in parentheses. All values in kilojoules. Biomass converted from grams using 20 950 J g\, assuming the bulk of the preserved prey are bivalves.

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a limited bias in preservation, the data suggest that non-preservable prey must have contributed disproportionately to the assemblage, and, indeed, examination of the life assemblage shows this to be true. 4.4. Differences between the life and death assemblages In every case, the life assemblage was dramatically different from the death assemblage. Taxonomically, this was caused by a preponderance of soft-bodied individuals that were not preserved. However, the same was true using tier and guild analysis. The life assemblage principally consisted of low-level epifauna and infauna, deposit feeders and predators. The death assemblage typically contained a much greater variety of tier and guild types in a more even representation reflecting the wider range of adaptive forms in the Mollusca. In most cases, the preservable life assemblage also was distinctive from the death assemblage, using numerical abundance. The preservable life assemblage had many more infauna and deposit feeders. Station 3, the deepest station, showed the best agreement between the preservable life and death assemblages. Station 1, the shallowest station, showed the worst. Staff et al. (1990a) suggested that Station 1 was affected more by shell transport which might reduce the similarity between the preservable life and death assemblages; however it is an enrichment of predators in the death assemblage at Station 1 that discriminates its population structure from that of the preservable life assemblage and it is not obvious how shell transport might selectively enrich this assemblage in predators. For tier structure, agreement between the preservable life and death assemblages was also poor using paleoproduction or paleoingestion. Infauna dominated the life assemblage to a degree not observed in the death assemblage, by paleoproduction, and tier structure was fundamentally different by paleoingestion. Guild structure was more similar in many cases. By paleoingestion, for example, the preservable life assemblage of Station 1 was similar to its death assemblage. By paleoproduction, the preservable life assemblage of Station 2 was similar to its death assemblage. However, overall, at none of the three stations did these broad community attributes compare well between the preservable life assemblage and the death assemblage. The assumption is that the preservable life assemblage varies substantially in this environment over a time span that is short in comparison to the time span represented by the top 15 cm of the stratigraphic record. Tier and guild structure of each of the three stations was distinctive even though the sites were nearby and covered a relatively small depth range. Infauna were more important in deeper water. Predators were more common in shallow water; deposit feeders were more common in deeper water. This small-scale variability probably originates from relatively small changes in the environment of deposition. Analysis of taphonomic signature for the three stations was reported by Staff and Powell (1990a). Each station was found to also have a distinctive taphonomic signature. Detailed reconstruction of both community attributes and taphonomic signatures of the death assemblages identify relatively small differences in the biological/physical/ chemical environment over a few kilometers distance in this area of the inner Texas shelf.

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4.5. The bay-to-slope transect Powell et al. (1992) described the tier and guild structure of an estuarine site in Copano Bay, Texas. Two assemblages were present at this site, one dominated by heterotrophic filter feeders and one dominated by bivalves (lucinids) with chemoautotrophic bacterial symbionts. Callender and Powell (1997) described a series of continental slope assemblages off Texas and Louisiana. Some associated with petroleum seeps were dominated by lucinids and thyasirids, each bearing chemoautotrophic bacterial symbionts. One, characteristic of the typical continental slope, was dominated by heterotrophic species. Together these assemblages, plus the three described here cover a transect from a variable salinity estuary to the upper continental slope and a diversity of assemblage types. This transect is characterized by a reduction in the size of the largest taxa that follows the offshore reduction in primary productivity characteristic of this part of the Gulf of Mexico (El-Sayed, 1972; Biggs and Sanchez, 1997). The Copano Bay assemblages contain taxa that routinely exceed 50 mm in size, whereas few of the heterotrophic slope taxa exceed 25 mm in size, with the exception, of course, of the deep-water seep communities that have an independent trophic source, chemosynthesis, that permits the presence of taxa of larger size. 4.5.1. Taxon richness Taxon richness was highest on the inner shelf (Table 9) and lowest on the slope. The tendency for taxon richness to increase in deeper water (Flint and Holland, 1980) was not observed in the death assemblage. Among the heterotrophic assemblages, taxon richness was about twice as high in Copano Bay as on the slope and another factor of 2 higher on the shelf. Within depth zones, taxon richness was more similar than between depth zones, despite the inclusion in two of three cases of assemblages with highly divergent guild structures, heterotrophic vs. chemoautotrophic dominance.

Table 9 Taxon richness and predator overrepresentation, based only on whole-shell data expressed as the ratio prey paleoproduction/predator paleoingestion Location

Assemblage

Taxon Richness

Ratio

Copano Bay, Texas

Heterotrophic Assemblage Chemoautotrophic Assemblage Station 1 (shallow) Station 2 (middle) Station 3 (deep) Lucinid Assemblage Thyasirid Assemblage Heterotrophic Assemblage

60 73 138 106 103 57 2 30

4.70 7.62 0.02 0.07 0.58 30.58 R 0.006

Texas Inner Continental Shelf Continental Slope

Note: Predators are not included in prey paleoproduction in calculating the ratio prey paleoproduction/predator paleoingestion, even though they may consume each other occasionally. All values in kilojoules. Biomass converted from grams using 20 950 J g\, assuming the bulk of the preserved prey are bivalves.

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The exception was the thyasirid assemblage which, even among chemoautotrophic seep assemblages, was unusually low in taxon richness (Callender and Powell, 1997). 4.5.2. Tier structure By abundance, tier structure varied from near complete dominance by infauna in some cold-seep assemblages to epifaunal dominance in Copano Bay (Fig. 4). The three inner shelf stations resembled the heterotrophic slope assemblage more closely than any other. In all nine cases, utilization of just the whole-shell component of the assemblage did not markedly change tier structure. The largest change came from the lucinid assemblage on the continental slope where the proportion of high-level epifauna was higher in the whole-shell component of the assemblage. Using paleoproduction, a community attribute based on size as well as number, markedly changes this picture. Most assemblages are now dominated by infauna. Exceptions include one inner shelf assemblage and one Copano Bay heterotrophic assemblage that retain a more diverse tier structure and the heterotrophic slope community where infauna are now rare. Infaunal dominance in Copano Bay and in the cold-seep assemblages originates from the presence of large infaunal bivalves in the assemblage. That is not true on the shelf. The shelf and heterotrophic slope assemblage have few large animals and two of the four show a trend away from infaunal dominance. Thus, for the most part, although not exclusively, infaunal dominance in paleoproduction originates from the presence of large bivalve mollusks. Using paleoingestion, a community attribute based on age as well as size and number, continues this trend away from infaunal dominance. The seep assemblages remain dominated by infauna, however, two of three shelf assemblages, the heterotrophic slope assemblage, and one of two heterotrophic bay assemblages are now dominated by epifauna or semi-infauna. In Copano Bay, grazers and low-level filter feeders are primarily responsible. Offshore, predaceous gastropods are the primary contributors. Overall, numerically, within-habitat (alpha) variability is somewhat less than between-habitat (beta) variability, particularly excluding the thyasirid assemblage. By paleoproduction and paleoingestion, however, within-habitat variability is at least as high as between-habitat variability. Thus, considerable small-scale variability exists in the contribution of large and long-lived animals to these assemblages. The numerically abundant and more opportunistic smaller animals are much more conservative in their tier structure, at least within habitat. 4.5.3. Guild structure By abundance, guild structure varied widely. Some seep assemblages were dominated by chemoautotrophs. Other assemblages were dominated by deposit feeders or grazers and filter feeders (Fig. 5). The three inner shelf stations were unique in the large proportion of deposit feeders present. The Copano Bay assemblages were unique in the number of grazers and low-level filter feeders present. Predators were common on the continental slope in the heterotrophic assemblage and also on the inner shelf. Perhaps most surprisingly was the absence of an assemblage that was truly dominated by filter feeders, even in Copano Bay, and the rarity of assemblages

Fig. 4. Comparison of life-habitat strategies for the fauna of selected death assemblages covering the estuary-to-slope transect of the western Gulf of Mexico using habitat tier categories, listed as percentages of the assemblage evaluated by total numerical abundance, whole-shell abundance, paleoproduction, and paleoingestion. Calculations of paleoproduction and paleoingestion based on the whole-shell component of the death assemblage only.

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Fig. 5. Comparison of feeding strategies for the fauna of selected death assemblages covering the estuary-to-slope transect of the western Gulf of Mexico using a complex set of guild categories, listed as percentages of the assemblage evaluated by total numerical abundance, whole-shell abundance, paleoproduction, and paleoingestion. Calculations of paleoproduction and paleoingestion based on the whole-shell component of the death assemblage only.

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dominated by deposit feeders. In all nine cases, utilization of just the whole-shell component of the assemblage did not markedly change guild structure. The largest change came from the Copano Bay assemblages where the proportion of deposit feeders increased relative to the filter feeders and on the shelf where the grazer component was minimized by use of the whole-shell component of the assemblage. Using paleoproduction, a community attribute based on size as well as number, markedly changes the guild structure of all assemblages except those from the continental slope petroleum seeps and those on the inner shelf. In Copano Bay, the large bivalves control guild structure; low-level filter feeders dominate the heterotrophic assemblages; chemoautotrophs dominate the chemoautotrophic assemblage. In the heterotrophic slope assemblage, predators become dominant. Overall each location is unique in its guild structure. Using paleoingestion, a community attribute based on age as well as size and number, modifies this picture in only one case. On the shelf and on the heterotrophic slope, predators become dominant. Primary consumers, either large chemoautotrophic or filter-feeding bivalves, dominate in Copano Bay and in all chemoautotrophic assemblages (bay and cold seep). Overall, numerically, within-habitat variability is much less than between-habitat variability, with the exception of the chemoautotrophic assemblages which are uniquely similar on the slope and in Copano Bay. Thus, guild structure is more conservative than tier structure on the small scale. By paleoproduction and paleoingestion, this trend remains with one exception. The dominance of predators is a unique attribute of all heterotrophic slope and shelf assemblages. With the exception of this similarity between heterotrophic slope and shelf assemblages, the trophic composition of the long-lived, mostly larger individuals is a unique attribute of these assemblage types; and, as uniquely different as the numerically abundant, mostly smaller species that dominate numerical abundance. Finally, the rather dramatic change in guild structure between numerical abundance, paleoproduction and paleoingestion suggests that, except for the cold-seep assemblages, the trophic composition of the smaller, numerically dominant taxa is significantly divergent from the larger and longer-lived species. This is particularly true when comparing the numerically dominant taxa to the longer-lived species that dominate paleoingestion. 4.5.4. Paleoguild structure For paleoguild structure, we simplify the guilds by placing the chemoautotrophs (with the exception of the vesicomyids — Callender and Powell, 1997) into the filter-feeding guild and combining the grazers with the deposit feeders. By abundance, paleoguild structure was distinctly less variable across habitats than was guild structure. Assemblages fell into two groups, those dominated by deposit feeders and those dominated by low-level filter feeders. The deposit-feeder dominated assemblages included the Copano Bay and inner shelf assemblages. The filter-feeder dominated assemblages included all of the slope assemblages (Fig. 6). Predators were common on the continental slope in the heterotrophic assemblage and also on the inner shelf. The surprising rarity of deposit-feeder and low-level filter-feeder dominated assemblages when evaluated by guild structure is now reversed, with these two

Fig. 6. Comparison of feeding strategies for the fauna of selected death assemblages covering the estuary-to-slope transect of the western Gulf of Mexico using a simplified set of paleoguild categories, listed as percentages of the assemblage evaluated by total numerical abundance, whole-shell abundance, paleoproduction, and paleoingestion. Calculations of paleoproduction and paleoingestion based on the whole-shell component of the death assemblage only.

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guilds accounting for most of the individuals in each of the nine assemblages. In all nine cases, utilization of just the whole-shell component of the assemblage did not markedly change paleoguild structure. The largest change came from the inner shelf assemblage where the deposit-feeder component was reduced by use of the whole-shell component of the assemblage. Using paleoproduction, a community attribute based on size as well as number, markedly changes the paleoguild structure of all assemblages except those from the continental slope thyasirid assemblage and those on the inner shelf. In the Bay assemblages, the large bivalves control paleoguild structure and the proportion of low-level filter feeders increases. In the heterotrophic slope assemblage, predators become dominant. Overall each location is unique in its paleoguild structure, unlike the case where numerical abundance was used. Using paleoingestion, a community attribute based on age as well as size and number, modifies this picture significantly on the inner shelf and the heterotrophic slope. On the shelf and in the heterotrophic slope assemblage, predators become dominant. The cold-seep assemblages and Copano Bay assemblages are not markedly changed, although deposit feeders are somewhat more important in Copano Bay than they were using paleoproduction. [This latter might be an artifact of the uncertain age of the lucinids — Callender and Powell (1997).] 4.5.5. Guild and tier structure — overview To summarize, numerically, within-habitat (alpha) variability is considerably less than between-habitat (beta) variability, with the exception of the two lucinid (chemoautotrophic) assemblages which are uniquely similar on the slope and in Copano Bay. By paleoproduction and paleoingestion, this trend remains with two significant exceptions. First, the Copano Bay and continental slope seep assemblages differed only in the slightly higher proportion of deposit feeders in Copano Bay. These assemblages are dominated by relatively large primary consumers (chemoautotrophic and heterotrophic bivalves). Second, the inner shelf and heterotrophic slope assemblages are dominated by predators, particularly when measured by paleoingestion. Finally, the rather dramatic change in guild structure between numerical abundance, paleoproduction and paleoingestion suggests that, except for the seep communities, the trophic composition of the smaller, numerically dominant taxa is significantly divergent from the larger and longer-lived species. This is particularly true when comparing the numerically dominant taxa to the longer-lived species that dominate paleoingestion. 4.5.6. Predator overrepresentation The relative proportion of predators on the shelf and slope heterotrophic assemblages is striking. Comparing predator lifetime ingestion (paleoingestion) with prey lifetime production (paleoproduction) reveals that the cold-seep and Copano Bay assemblages have a large surplus of primary consumers, regardless, in Copano Bay, of whether the assemblage is dominantly heterotrophic or chemoautotrophic (Table 9). In sharp contrast, the heterotrophic slope assemblage and the three inner shelf assemblages are overrepresented by predators. Assuming that these proportions

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indicate the relative importance of non-preservable prey, non-preservable prey were relatively more abundant on the shelf and slope. This conforms with Scott’s (1978) initial supposition based on the examination of abundance data from a large number of bay and shelf assemblages. Potentially, it is characteristic of non-chemoautotrophic euhaline assemblages in general, however this will have to be tested using a wider variety of assemblage types. Onshore-offshore trends in predation have been suggested by trends in repair frequencies (Oji, 1996) and predator abundances (Scott, 1978). This analysis of predator overrepresentation demonstrates the risk of using predator abundances to examine trends in predation intensity, because predator abundance may be biased by a depth-dependent trend in the contribution of preservable predators to the predator complement of the life assemblage.

5. Conclusions Some community characteristics are more reliable or less affected by taphonomic processes than others. As found in previous studies (Parsons and Brett, 1991), most of the potentially preservable taxa were preserved in these inner-shelf death assemblages; however many more taxa were preserved than found living. The rank orders of abundance, paleoproduction, and paleoingestion of the preservable taxa in the life assemblages were greatly altered in the death assemblage. Overall, the life and death assemblages did not agree well at either of the three inner shelf sites, in contrast, for example, to some bay communities (Staff et al., 1985; see also Parsons and Brett, 1991). This is very likely due to the longer time represented by these shelf death assemblages and is probably generally true for shelf and slope assemblages, although much additional data will be required to verify this supposition. Analytical effort required for evaluating death assemblage composition is increased by at least a factor of two if fragments are included. An overriding issue, then, is the cost-to-benefit ratio of including fragments in the analysis. A review of nine different assemblages covering a wide spectrum of Texas bay-to-slope assemblages suggests that, at least in this region, inclusion of fragments, though enriching the analysis, does not add measurably to the conclusions derived from the analysis. Of course this conclusion can only be based on the community attribute measures derived from numerical abundance data, but, at least at that level, the whole-shell component of the assemblage was adequate for a thorough analysis of assemblage guild, tier, and taxon structure. The assemblage types were each unique in a combination of key abundance, paleoproduction and paleoingestion-derived community attributes. Using the three approaches simultaneously significantly enriched the evaluation of these assemblages and provided important discriminating characteristics. At least as important, however, were the resemblances between assemblages. All shelf and heterotrophic slope assemblages were similar in their predator dominance of paleoingestion. Deposit feeders and chemoautotrophs increased in importance numerically offshore, but not

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when evaluated by energy flow. All offshore assemblages were characterized by 40% or more of the individuals being infaunal. The relative importance of epifaunal predators, which might reduce the abundance of epifaunal prey in the heterotrophic assemblages, probably explains infaunal dominance at these sites. At the petroleum seeps, the explanation probably lies in the nature of the taxa that harbor chemoautotrophic bacterial symbionts, which are dominantly infaunal clams. [We have not considered mussel-dominated seep sites because of the minimal likelihood that these assemblages will be recognizably preserved (Callender et al., 1994)]. On the whole, tier structure was more variable than guild structure within habitat. On the whole, paleoingestion was more variable than numerical abundance or paleoproduction, probably because of the reliance of paleoingestion on long-lived taxa that are normally relatively rare. Greatest similarity was seen in paleoguild structure within and between habitat. Bay, shelf and slope assemblages were similar in many respects using this approach. Taxa identified as deposit feeders in paleoguild analysis were abundant in all but two assemblages. Taxa identified as low-level filter feeders were dominant in both bay and slope assemblages. The assemblages overrepresented by predators could still be discriminated, however chemoautotrophic assemblages were not unique, nor were shelf assemblages unless paleoingestion was used. The use of simplified guild categories, paleoguilds, clearly limits the discrimination of assemblage types and the identification of depth-related trends in assemblage structure. Taphonomic signatures constructed in previous studies indicated large withinhabitat variability between nearby locations on the shelf and slope (Callender et al., 1992; Callender and Powell, 1992; Staff and Powell, 1990a). With a few exceptions, notably among the seep assemblages evaluated numerically, within-habitat variability was low in comparison to variability between habitats in tier and guild structures and taxon composition. The three inner shelf assemblages were relatively similar in comparison. The three bay assemblages were similar numerically and the two heterotrophic assemblages retained that similarity using energy flow measures. The degree of within-habitat variability in taphonomic processes in the bay and on the inner shelf was not adequate to significantly modify assemblage structure on the small scale (and, of course, the life assemblages must have been similar). Not so in the cold-seep habitats where rates of taphonomic loss are likely the highest of any location covered by this study (e.g. Staff et al., 1986; Cummins et al., 1986; Staff and Powell; 1990a; Callender et al., 1994; Callender and Powell, 1997). Very likely, at these sites, a significant component of assemblage structure is determined by taphonomic processes rather than the ecology of the living communities.

Acknowledgements This study was funded by NSF grants EAR-8506043, EAR-8803663, and EAR9218530 and a contract from the Minerals Management Service (14-12-0001-30555). Submersible support was received from the NOAA-National Undersea Research Program (Johnson-Sea-¸ink) and the U.S. Navy (NR-1). We would like to thank the

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