Benthic infaunal variability on a transect in the Gulf of Mexico

Estuarlne and Coastal Marine Sdence (x98o) Io, x-I4

B e n t h i c Infaunal Variability on a Transect in the G u l f of Mexico

R. Warren F l i n t a n d J. S e l m o n Holland University of Texas, ~Iarine Science Institute, Port Aramas 2tlarine Laboratory, Port Aransas, Texas78373, U.S.A.

benthic fauna; environmental changes; species diversity; sediment distribution; Gulf of Mexico

Keywords:

Macroinfaunal benthos off the'South Texas Coast of the Gulf of Mexico formed different assemblages distributed according to depth: shallow (22 m), mid-depth (36 to 49 m) and deep water (78 to x3x m). Species composition of shallower stations were less diverse, composed of eurytopie and opportunistic species adapted to a fluctuating environment. The deep water benthos, in a more stable environment, had a higher diversity. Sediment composition (high proportions of silt) at the mid-depth stations resulted in dominance of deposit feeders. The environmental gradient was related to a species continuum which changed from polychaete dominated groups in shallow water, to deposit feeding molluscs and crustaceans, to a deep water diverse fauna not dominated by any particular group. Environmental heterogeneity, including climatic variability, may be most important in controlling shallow water benthos. In deeper more stable shelf habitats where diversities are higher and species equilibrium is the case, species interactions may determine community structure. Introduction

Since Petersen (I9X3; x9xS), investigators have delineated benthic communities in relation to environmental parameters such as hydrological variables (Molander, i928), physical properties of the bottom sediments (Jones, x95o), and biological adaptation derived from species interactions in relatively stable environments (Sanders, x968). Community distributions vary considerably in space (e.g. Lie & Kelley, x97o; Johnson, x97o; Field, I97X; Boesch, x973; Flint & lV[erckel, 1978) due in great part to the general heterogeneity of aquatic systems and the tendency towards patchiness in benthic fauna. The above studies, however, have reported gradients in many of these community properties, the causes of which have been the subject of considerable speculation (Whittaker, I975). ~iueh of this speculation has been concerned with the role of environmental variability such as bottom sediment spatial vafiabilit3? (e.g. Day et al., x97x ; Tenore, i972 ) or climatic irregularity (e.g. Sanders et al., x965; Hessler & Sanders, x967). An often-used technique taken to examine the effects of environmental variation on community structure is to sample communities along some physical gradient. Although some gradients may involve variation in a single factor such as temperature (Kullberg, x968), marine benthos is rarely that simple but will usually reflect integrated effects of environmental parameters. The many factors contained in such gradients are assumed to vary in some reasonably consistent manner (Rotenberry, x978). University of Texas Marine Science Institute Contribution No. 328

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This research defines the benthic communities on a transect off the south Texas coast representing environmental gradients, in a little studied area of the Gulf of Mexico. Gradients and community structure of the benthos wcrc determined to allow for comparison with faunal-environmental studies in other marine systems. Methods Six stations on a transect on the outer continental shelf of the Gulf of Mexico (Figure x), depths ranging from 22 to x3x m, were sampled on x2 cruises between January r976 and September x977. During each collection bottom water salinity and temperature were measured. Six grab samples at each station were used for species enumeration and sediment textural analysis. Macroinfauna was sampled with a Smith-Mclntyre (I954) bottom sampler which sampled a o.r m"- surface area to a maximum depth of approximately z7 cm. A small portion of the sediment was removed from each grab with a Plexiglass core (5 cm diameter) for textural analysis. The remainder of the grab was washed through o. 5 mm mesh. The screened remains were relaxed in a saturated magnesium sulfate solution, and preserved with IO~/oseawater-formalin containing Rose Bengal as a vital stain. Sediment texture was analyzed by the rapid sediment analyzer (Schlee, i966 ) for the sand sized fraction and by the pipette method (Folk, x974) for the mud fraction. Interpretations for the proportion of sand were done graphically at each o.25 phi (r interval and used to calculate moment and graphic grain size parameters by standard methods (McBride, x97Q. In the laboratory, the fauna was sorted from the screened debris into major groups. Organisms were identified to species or lowest possible taxon and counted. Taxa not identified to species were referred to by their generic names and usually distinguished by an appropriate code (e.g. sp. A, sp. B, etc.). The measure of species diversity for the species' lists at each station during each sampling interval was calculated by the Shannon-Wiener diversity index (Piclou, I966 ) using logx0, equitability (Lloyd & Ghelardi, 1964), and Hulbert's (z97r) probability of interspecific encounter (P.I.E.). By combining a diversity index with measures of richness (number of species) and evenness (distribution of relative abundance of the species) and providing the P.I.E. measure, a reasonable comparison between communities could be accomplished. Species composition of individual stations were compared using community ordination (Oriocci, I96I ). This technique used the similarity measure and axis construction method introduced by Bray & Curtis (x957). Log transformation (log N + I ) was done on the abundanccs in order to decrease the effects of a few species with large abundances. Both the R and Q techniques (Sheath & Sokal, 1973) of ordination were applied to the infaunal data. In addition, because there were a number of rare species (25-3O~/o) that contributed very little information to the final results of ordination, these were removed from the analysis by a selection of 'biologically important' species (Thorson, i957). The working definition of 'biologically important' was that the species represent at least 2~o of the total infaunal abundance at a station for one sampling period or that they occur on at least 25% Of the stations sampled during an interval. Analysis of variance was used to test for differences between community properties; mutiple regression analysis was used to identify environmental-infaunal relations. In addition, the coefficient of variation was used to measure the stability or persistence of an individual species or the total fauna at a given station over time. A low coefficient of variation meant that numerical densities for that species remained relatively constant over the time ~

4

2~. IV. Flint H ft. ,~. Holland

period considered (species in equilibrium). Ahematively, a high coefficient of variation denoted considerable numerical variation or instability for the species over the same time interval (nonequilibrium condition).

Results Physical characteristics Bottom water salinity and temperature changed gradually with increasing water depths along the transect. There was a decrease of variation in temperature and salinity in an offshore direction (Table x). Oneway analysis of variance indicated that the change in these variables with depth was significant (P
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Several different properties of sediment texture were measured for each station on the transect and the means of these parameters also illustrated a gradational pattern with depth (Table x). Comparison between replicates at Station 6 displayed a silty (3o%) clay of very uniform texture, with the finer particles better sorted than the coarser particles. A slightly coarser silty clay occurred at Station 4 and 5 of the transect, and finer particles were again better sorted than the coarser ones. There was quite a variable midshelf textural mixture exhibited by the samples from Stations 2 and 3. These stations showed the highest silt content and exhibited high sorting coefficients and skewness suggesting the coarser particles were better sorted than the finer particles. Station x was comprised of sandy muds with a medium particle diameter of 7"43 ~ (Table x). The sorting coefficient was high at 3.4 z ~ and the highest percentage of sand was also observed at this site. Analysis of variance for all sediment parameters illustrated in Table x showed that significant differences (P
Species number, in.faunal density, community characteristics of diversity, probability of interspecific encounter (P.I.E.) and equitability all showed several substantial changes from shallow to deep water along the transect (Figure 2). Although there was a significant variation of species number (P
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6

R. IV. Flint &.7. S. Holland

indicated by the 95% confidence intervals was much greater at the deeper transect stations. In contrast, with the exception of Station 5, infaunal density showed greatest variation at the shallowest sites. Species diversity, P.I.E., and equitability measures were significantly different (P
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were observed at Stations 4, 5 and 6, and were primarily characterized by molluscs and crustaceans. The species in Group V were found consistently only at Station 6, again including primarily species other than polychaetes. According to Figure 4 all the species listed exhibited significant differences in densities between the six stations. Although this was also true for the ubiquitous species (Group I), based on results contrasting densities, the pattern for this group did not present a true picture because total infaunal density of the community decreased with depth (Figure 2). The first four species of this group were often the dominant fauna at all stations during the study period. Contrasting the per cent of the total density that these four fauna represented over all stations showed that there was not a significant difference (P
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The two factor analysis of variance allowed us to evaluate species density differences for both. main effects of station and time, independent of one another. The main effect of time on the species densities indicated that x8 species did not show differences on a seasonal basis. Twelve of the x8 species occurred only at the deeper stations on the transect and another four, )IIediomastus californiesis, Lumbrineris parvapedata, gtmpelisca agasslzi, and Nerdd (Nicon) sp. A, were some of the most abundantspecies at the shallowest sites. The remaining species that did show significant density differences over time in many instances also exhibited significant interactive effects with station. Therefore nothing conclusive could be drawn from these fauna. To better define temporal variation in terms of all species at each station, coefficients of variation for each species at each station were plotted in order of increasing value (Figure 5), according to a method described by Sanders (x978). A species had to be present at a mean density of greater than 2 m-2 in order to be included in the analyses for that station. Largest coefficients were present ~t Station 2 for the majority of species (Figure 5). Station 3 also exhibited extremely high coefficients after the 7o% ranking. The shallowest and deepest sites on the transect, however, consistently exhibited the lowest coefficients of variation. The null hypothesis stating that the coefficients of variation for the suite of species having mean densities greater than 2 m-2 at one station were not significantly different from those species' coefficients at another station was tested by comparing mean differences. This test showed that the stations were not significantly "different from one another at a P=o.x34. Although the null hypothesis was not rejected the curves did indicate that the species stability appeared to be less at Stations 2 and 3 on the OCS transect.

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Env#omhental-faunal relationships

A summary of the correlations that e~sted between the general community parameters artd environmental variables measured during this study are illustrated in Table 3. Very little relationship was observed between any independent variable and the number of species observed at each station. The highest correlation, variation in bottom water temperature (Temp STD) explained only 9% (r2) of the species number changes spatially. Salinity standard deviation explained 57% of the variation observed in infaunal density while an additional z % of density variation was explained by the next strongest variable, silt, according to multivariate regression analysis (Table 4).

Benthic infaunal variability, Gulf of Mexico

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TABLE 3. Significant correlation coefficients (P
Density

Diversity Equitability

P.I.E.

Ordination Coordinate x

Ordination Coordinate 2

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0"38 --0.53

0"44 0"57 0"23 0"75 --0"58 0"76 --o'65

--0"46 --o'5t --0"36 --0"79 o"4x --0"78 o'67

--0"59 --0"62 --0"36 --o.8x 0"49 --0"79 o'75

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TABLE4. lX{ultipleregression results of the dependent benthic community variables against the independent environmental factors. The independent variables and their percent explained variation are listed in order of entry into the stepwise regression analysis. Only significant (P
Significant independent variable and explained variation in dependent variable

Density Diversity Equitability P.I.E. Coordinate x Coordinate 2

Sal STD (57%) Temp. STD (63%) Temp. STD (66%) Sal STD (58%) Temp. S T D (87%) Silt (35%)

SILT (2%) TEMP (2%) Depth (3%) TEMP (2%) TEMP (9%) TEMP (x%) Sal STD (3%) Sal STD (5%) Temp STD (29%)

Species diversity and equitability were both highly correlated with bottom water temperature standard deviation (Table 3). The variation explained for each. dependent variable was 63 and 66%, respectively. According to multiple regression results, both dependent variables had another 2% of their variation explained by bottom water temperatures (Table 4). An additional 3% of variation in species diversity was explained by depth which suggested that additional depth-related factors not measured here may have an effect on benthic species diversity. The benthic community parameter P.I.E. was highly correlated to salinity standard deviation of the bottom waters (Table 3). l~Iultiple regression further illustrated that temperature, depth and per cent silt explained an additional 13% of the variation in this parameter (Table 4). To this point, most of the standard benthic community measures have shown strongest relationships with factors that suggest environmental variability may be influencing these communities. Correlation of the first coordinate from community ordination with the environmental variables fdrther confirmed the above observation. The highest correlation for this variable was with temperature standard deviation (Table 3). The second ordination coordinate, however, e.'-daibitedthe greatest correlation with silt. Graphical representation of these correlations in respect to the actual ordination plots is shown in Figure 3. Multiple regression analyses (Table 4) illustrated that water temperature and salinity standard deviation explained an additional 4% of the variation in the first ordination coordinate. It was further observed "that the residuals of these independent variables explained another 34% variation in the second ordination coordinate, after the partial correlation effects of silt were removed (Table 4).

x2

R. IV. Flint &.,~. ~. Holland

Discussion Analysis of the benthic infauna along an outer continental shelf transect in the Gulf of Mexico indicated that there are several species assemblages. Frankenberg & Leiper (i977) suggested that the typical terrestrial concept of gradient analysis (Whittaker, x967) may be useful for interpreting certain dynamics of subtidal benthos. The change in environmental variables on the transect in this study provided an ideal opportunity to examine the benthos in respect to gradients. The results of community ordination (Figure 3) best exhibited the prominent environmental-infaunal patterns of the study area. From these results it was confirmed that a continuum existed in which the sampling sites were arranged in a multi-dimensional framework of composition gradients based on the degree of biotic relationships among sites. The biotic information contained in the first coordinate was clearly divided between three station groups which were directly related to decrease in hydrologic variability. The shallowest site was distinguishable by a temperature standard deviation greater than 5.o~ over the study interval. In contrast the mid-depth stations (z and 3) were in a middle range of the temperature variation and the three deepest sites (4, 5 and 6) occurred in a relatively stable bottom water habitat. Five species groups characterized the station groupings and these appeared to blend into one another along the gradient (Figure 4). The second principal component (Figure 3) was further able to differentiate the mid-group stations from the others in terms of biotic information. This distinction was directly related to the stations showing the highest silt content on the transect. These statons also exhibited the highest skewness measures for their respective sediments which in conjunction with the high silt content suggested the presence of a very ill-sorted homogeneous sediment habitat that was relatively unstable because of the amount of silt. Another interesting characteristic of Stations 2 and 3 was the apparent instability of species' densities (Figure 5). As shown by the coefficient of variation plot, the greatest coefficients were usually observed at these sites. We hypothesize that the unstable silty sediments at these sites were more easily disturbed by storm-wave scour and bottom currents than the other study sites, especially the deeper sites with better binding clays (Sanders, i958). Thus the densities of individual species were more unstable at the mid-depth stations because of more periodic disturbances to the sediment in which they lived. This hypothesis is further supported by considering some of the fauna present at these sites and the functional modes they represented. These sites were characterized by fewer bivalves than the deep stations (Table 3 and Figure 4), especially those that are filter-feeders. The deposit-feeding polychaetes were numerous (i.e. Cossura delta, M. californiensis, and Magelona longlcornis) and there were a number of small crustaceans that also existed primarily by deposit-feeding (i.e. Eudorella monodon and A pseudes sp. A). A model proposed by Tramer (x969) for patterns observed in components of species diversity along environmental gradients also fit the observations in this study. Tramer expected variations in sl~ecies richness to Occur in stable, non-rigorous environments with variations in evenness to occur under the opposite conditions. It is apparent from Figure 2 that although there was no significant difference between Stations 2-6 for species diversity, the diversity components of rich~.~7 and evenness (equitability) did show different trends. The results strongly suggested that the number of individuals (species density) became increasingly concentrated in a smaller proportion of the species as one moved from more stable, deep waters to the mid-shelf stations. In other words evenness decreased. In addition, richness was much more variable at the deep sites as illustrated by the confidence limits, while evenness was more variable as depth decreased.

Benthic infaunal variability, Gulf of Mexico

x3

The shallower collections consisted primarily of polychaetes (Table 2 and Figure 4) many of which were dominant members of the community and multi-voltlne in terms of life history pattern. These polychaetes, especially Paraprionosplo pinnala, Mediomastus californiensis, and Magelona phyllisae are found in a variety of environments including estuarinc waters (Poff & Holland, manuscript in preparation) and are opportunistic in nature. These populations are able to exploit favorable conditions causing the community equitability to fluctuate with the changing environmental conditions. In contrast the deepest stations supported almost as many crustaceans and bivalves as they did polychaetes. Many of these species are uni or bi-voltine, reproducing usually in the spring and]or late summer (i.e. Pitar cordatus and Th)'asirapygmaea). These populations are more in equilibrium as indicated by the fewer significant changes over time contrasted to the shallower populations (Figure 4). Therefore, in fitting the model proposed by Trainer, the benthos diversities of the outer continental shelf arrayed along a gradient of environmental stability changed in response to changes in evenness. The data presented above are consistent with the hypothesis that environmental heterogeneity, driven in part by bottom water variability, may be relatively more important in structuring the benthos in the shallow continental shelf waters of the Gulf of Mexico. In contrast with decreases in this heterogeneity, competitive interactions between species may be more important in determining community structure and allowing species equilibria to be maintained. We conclude from this study that the South Texas OCS subtidal benthos is comprised of macroinfaunal species that follow a continuum from shallow to deep water. This continuum is characterized by species assemblages that blend into one another along an environmental gradient. These assemblages slowly change from polychaete dominated groups to communities characterized by deposit feeding molluscs and crustaceans and finally to a very diverse fauna not particularly dominated by any particular taxa. Sediment texture plays an influential role in dictating where certain species can best function due to their strategies. The most influential factor that distinguishes the communities at either end of this continuum, however, appears to be bottom water variability.

Acknowledgements We thank E. W. Behrens for sediment textural analyses and S. Holt, C. Nicol, R. Yoshiyama, D. Wohlschlag and C. A.~nold for their critical reviews of the manuscript. The graphics were done by Tom i~Ioore'ancl typing by Helen Garrett. Additional gratitude is extended to all those who worked on the taxonomy of these organisms. This work was supported by the Bureau of Land ~,!Ianagement, Contract AA55o-CT6--x7, to the University of Texas.

References Anderberg, M. R. x973 Cluster Analysis for Applications. New York, Academic Press. Boesch, D. F. x973 Classification and community structure of macrobenthos in the IIarnpton Roads Area, Virginia. ~larine Biology 2I~ 226--244. Bray, J. R. & Curtis, J. T. x957 An ordination of upland forest communities of Southern Wisconsin. Ecology JSXonographs27~325-349. Day, J. S., Field, J. G. & Nlontgomery, M. x97x Use of numerical methods to determine the distribution of benthic fauna across the continental shelf of North Carolina. ffournal of Animal Ecology 4o~ 93-xz6. Field, J. G. x97x A numerical analysis of changes in the soft-bottom fauna along a transect across False Bay, South Africa. ffournal of Experimental 2~Iarine13iology and Ecology 7~ 2x5-253.

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