- Email: [email protected]
Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea)
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea) M. T. Delafontaine,* B. W. Flemming and A. Bartholom/i
Senckenberg Institute, Schleusenstr. 39a, D-26382 Wilhelmshaven, Germany
ABSTRACT
From March 1995 to October 1996, seasonal export and import fluxes of POC in the mud and sand fractions of an intertidal sand flat (area: 1400,500 m - 0.7 km 2) were investigated in the vicinity of a series of M. edulis banks behind Spiekeroog Island (East Frisian Wadden Sea). In January/February 1996 the region experienced a severe ice winter, and together with typical a u t u m n / w i n t e r storms, the budget assessments presented in this paper are thus representative of major physical and biological disturbances common in the Wadden Sea. The contents and concentrations of mud in the sediments of the survey area varied considerably, being generally higher in late summer and lower at the end of winter, although almost the same basic pattern was retained over the 19 month survey period. The average POC content of the local sands was 0.106%, whereas that of the mud fractions varied in the range 1.5-4.0 wt%, lower values being associated with high mud contents, higher ones with low mud contents. Overall, POC concentrations in the mud fractions varied from <50-150 g m 2, although peak values of 300 g m 2 occurred locally. Between March and September 1995 a net import of 262 t POC was recorded. In the period September 1995 to April 1996, which included the severe ice winter, a net export of 477 t POC took place. In contrast to the previous year, the summer following the ice winter, i.e. the period from April to October 1996, was characterised by a net export of 83 t POC. In all cases, 40-44% of the total POC fluxes were associated with the sand fraction, in spite of the generally low POC contents of sand. This observation reveals an important and hitherto unrecognised role of sand in total POC fluxes in the Wadden Sea, a feature evidently linked to the large turnover of sand which contributed >90% to the total sediment flux. Contrary to widespread belief, sand does not necessarily dilute substances attached to the m u d fraction in sand-mud mixtures but can actually act as a concentration mechanism because it induces substantial changes in sediment bulk density. This feature will, amongst others, have to be taken into account in the feeding ecology of benthic marine organisms, deposit feeders in particular.
* Corresponding author: M.T. Delafontaine e-mail: [email protected]
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1. INTRODUCTION It has long been known that a significant proportion of the pelagic particulate organic pool can be channelled to the benthic realm, creating a tight coupling between these 2 compartments in both coastal and deep-sea ecosystems (e.g., Hargrave 1980; Graf 1989; Dame 1993). In many deep-water environments, hydrodynamic conditions are weak enough to facilitate the deposition of particles originating in the productive upper layers. Shallow coastal waters, by contrast, are generally more turbulent and, consequently, the bottom sediments are often impoverished in fine-grained material. On the back-barrier tidal flats of the Wadden Sea (southern North Sea), mud flats have generally disappeared as a result of land reclamation (Flemming & Nyandwi 1994). Because of this loss in accommodation space, the physically controlled deposition of fine particles is today largely restricted to a few remaining embayments which are sheltered enough. By implication, the exchange of particulate matter between the water column and the sediments has become increasingly dependent on biodeposition, i.e. the condensation by filtering organisms of suspended organic and inorganic material in pseudofaeces and faeces. In this respect, the mussel Mytilus edulis is arguably the most important species in the region (e.g., Smaal et al. 1986), its filtering activity resulting in the formation of characteristic 'mud halos' in otherwise sandy areas where hydrodynamic conditions usually preclude the deposition of fine particles (e.g., Flemming & Ziegler 1995). The extent to which biogenic muds can be used as, amongst other things, a food source by benthic organisms depends largely on the size and turnover rate of this mud reservoir. In the Wadden Sea, fine-grained sediments are easily resuspended under existing hydrodynamic energy conditions, particularly during the more stormy autumn and winter months from about September to March (e.g., Flemming & Delafontaine 1994; Ten Brinke et al. 1995; Bartholom~i et al. 2000, this volume). In addition, a substantial mixing of biogenic muds with coarser grained sediments occurs on these tidal flats (e.g., Oost 1995), and sediments in the mussel banks mostly contain less than about 20% mud on a dry weight basis (e.g., Delafontaine et al. 1996; Bartholom/i et al. 2000, this volume). For the East Frisian Wadden Sea, seasonal mass balances have, in fact, demonstrated that total fluxes of mud (i.e. import+export) generally constitute only about 6-7% of total sediment fluxes in M. edulis banks (Bartholom/i et al. 2000, this volume). Higher enrichment levels in fine-grained material would tend to counterbalance this apparent dominance of coarser material in, for example, the cycling of particulate organic carbon (POC). However, the findings of Delafontaine et al. (1996, 2000) imply that such POC budgets are neither simple nor straightforward for the region. Thus, monthly POC enrichment levels varied strongly in the mud fractions of sandy back-barrier tidal flats behind the islands of Baltrum and Langeoog in the East Frisian Wadden Sea. Furthermore, in the mud fractions of sandy sites devoid of mussels, POC contents were generally higher (3.5-5.5 wt%) than those recorded in the vicinity of M. edulis banks where values fluctuated in the range 1.5-3.0%. Mussel
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occurrence also undergoes strong seasonal and annual fluctuations in the Wadden Sea (e.g., Dankers & Koelemaij 1989; Nehls & Thiel 1993; Flemming & Delafontaine 1994). Depending on factors affecting mussel survival and/or establishment, sandy sediments would therefore contain variable propor-tions of differently enriched mud pools. From March 1995 to October 1996 we investigated large-scale (700~103 m 2) seasonal export and import fluxes of POC in the mud and sand fractions of an intertidal sand flat behind Spiekeroog Island (East Frisian Wadden Sea), choosing a site which incorporated a series of M. edulis banks. POC data were collected concurrently to those for topographic elevation and sediment composition (cf. Bartholom/i et al. 2000, this volume). Roughly in the middle of the study period, a prolonged spell of ice coverage (ca. 50 days in January-February 1996; Anonymous 1996; Giinther & Niesel 1999) resulted in extensive damage and displacement of the mussels as well as considerable erosion (Bartholom/i et al. 2000, this volume). Together with typical autumn/winter storms, these budget assessments are therefore representative of major physical and biological disturbances common in the region.
2. MATERIALS AND METHODS 2.1. Study area, study grid The fieldwork was carried out on the Swinnplate tidal flat in the rear of the barrier island of Spiekeroog in the East Frisian Wadden Sea (southern North Sea; Fig. 1). The site represents a typical sand flat (mud contents usually <2 dry weight%) dominated by the polychaete Arenicola marina. A mixed community of the mussel M. edulis and the tube-dwelling polychaete Lanice conchilega with associated biogenic mud accumulations was present at the time of study (Kurmis 1995; Delafontaine et al. 1996).
Figure 1. Locality map (dotted lines in inset indicate watersheds).
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A grid which remained fixed for the duration of the study period (March 1995October 1996) consisted of 90 sampling/measuring points at 100-m intervals spanning several mussel beds over a rectangular area of 1400,500 m (0.7 km2; Fig. 2). The positions of the 90 grid points were fixed by laser theodolite with an accuracy of
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2.2. Mass balancing Mass balances of export/import fluxes for mud, sand, and POC were determined for three consecutive seasons in the time intervals March-September 1995 (7 months, summer), September 1995-April 1996 (8 months, winter), and April-October 1996 (7 months, summer). 2.2.1. Terminology In the following we define some key terms as used in the present study. Seasonal: a time interval spanning winter or summer, and lasting 7-8 months. Sand: sediment size fractions 0.063-2 mm. Mud: sediment size fractions <0.063 mm. Sediment: unfractionated sand+mud. Dry bulk density: dry mass of sediment (desalinated) in a unit volume of water-saturated sediment. Content of a substance in the sediment: dry mass of the substance in a unit mass of water-saturated sediment. Concentration of a substance in the sediment: dry mass of the substance (desalinated) in a unit volume of water-saturated sediment. Dry weight% content of a substance in the sediment: (dry mass of the substance/dry mass sediment)*100. Export: flux from the study area (loss), reflecting net erosion in a given time interval. Import: flux to the study area (gain), reflecting net deposition in a given time interval. Total flux: export+import. Mass budgets (net fluxes): import mass-export mass. Flux units" tonnes dry sediment (sand, mud) or POC per unit area per season. Tonne or t: metric ton (= 1000 kg).
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2.2.2. Procedure
Topography was measured by means of precision levelling at all 90 grid points, using a laser theodolite (see above) to determine elevations to ca. 1 cm relative to the German topographic chart datum (NN = normal null). Measurements were carried out every 7-8 months in the period March 1995-October 1996 (Bartholom~i et al. 2000, this volume). Export/import fluxes of sand and mud were determined by calculating the differences between the topographic elevations recorded at the beginning and end of a given 7 to 8-month study interval, and then generating corresponding changes in wet sediment volume (Fig. 3). These data were used in combination with concurrent measurements of sediment composition (mud content), and a site-specific relationship between dry bulk density and mud content (see below) to convert volumetric changes into gains or losses of sand and mud masses (Bartholom~i et al. 2000, this volume). Gains and losses in sand masses were multiplied by the POC content of local sands in order to calculate corresponding changes in sand POC masses. In order to calculate the mud POC masses, gains and losses in mud masses were coupled with the site-specific mud POC contents measured at the end and at the beginning of each study interval, respectively. to determine BULK SEDIMENT FLUX erosion = loss = export deposition = gain = import
measure VOLUME CHANGE by repeated precision levelling by applying site-specific DRY BULK DENSITY, convert into BULK SEDIMENT MASS (gain or loss)
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Figure 3. Flow diagram of the procedure followed to measure export/import fluxes of mud, sand, and POC on the Swinnplate sand flat.
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2.2.3. Sediments Concurrently to the elevation measurements, surficial sediments were collected to depths of up to 5 cm at all 90 grid points, at least 5 subsamples being pooled in each case in order to reduce the probable sampling error by at least 50% (e.g., Krumbein & Pettijohn 1938). The samples were desalinated in the laboratory, and then wet sieved through a 0.063-mm mesh. The resulting sand and m u d fractions were oven dried at 65~ for 24 hours, weighed, and these values were then used to calculate the m u d contents of the sediments (e.g., Lewis & McConchie 1994). The empirical relationship between the dry bulk density and m u d content of the sediments used in the present study was extracted from Flemming & Delafontaine (2000), i.e. y = -0.7955892+2.3863045e ~-~1~5829277~where y - dry bulk density of the surficial sediment (upper 5 cm; mass (g) of desalinated dry sediment per cm 3 of water-saturated sediment), x = weight% of dry m u d content of the sediment, n = 337, r = -0.9847. 2.2.4. POC POC contents were determined in all mud fractions (mud POC content = [dry mass POC in m u d / d r y mass mud] * 100). Aliquots were ground by means of a pestle and mortar and, after acid leaching with HC1 fumes for 15-24 hours, the organic carbon was measured by means of an Hereaus CHNS analyser in each case (standard: acetanilide; reproducibility: +4.8%, n = 10; cf. Delafontaine et al. 1996). POC contents of whole (unfractionated) sediments were measured in additional samples collected on the Swinnplate and in nearby Jade Bay. In this case each sample was halved before desalting, resulting in 2 groups of complementary subsamples. One group served for the POC analyses after grounding in a mechanical mill, the other group for the determination of mud contents. This data set was used to estimate the mean POC content of local sands by means of regression analysis. 2.4. Data plotting Contour plots were computer generated by means of the software package SURFER (Windows Golden Software Inc. RT) using the distance interpolation procedure (e.g., Cressie 1991).
3. RESULTS
The variability of mud concentrations (i.e. volume-specific m u d masses) in the survey area are illustrated in Fig. 4. It is clearly evident that almost the same basic pattern was retained over the 19 month survey period, in spite of the fact that m u d concentrations varied considerably, being generally higher in late summer and lower at the end of winter. The similarity of the pattern was all the more surprising as there was also considerable variation in sediment turnover as documented by substantial local elevation changes (cf. Bartholom~i et al., this volume). Mud is thus either
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worked into the sediment to depths of up to several decimeters, or older m u d banks essentially occupying the same area are exhumed in the course of erosion.
Figure 4. Mud concentrations (volume-specific m u d masses) in the survey area over a period of 19 months (a: March 1195; b: September 1995; c: April 1996; and d: October 1996). Note the strong variability from survey to survey.
Regression analysis of POC contents in sediments of varying sand-mud proportions (ca. 0.5-65 wt% mud) shows that the POC content of local sands is 0.106%. Thus, y - 0.106+0.023x where y - POC content of sediment, x - m u d content of sediment, n - 55, r - 0.86.
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In spring (March) 1995, the POC contents of the mud fractions varied in the range 1.5-2.0 wt% in large areas of the Swinnplate grid (Fig. 5a). Higher values of 2.0-3.0% POC were recorded in the north-western and more centrally situated southern sectors. Contents had generally risen to 1.5-2.5% by the end of summer (September) that year, and higher values of 3.0-4.0% were again found at the north-west sites as well as more to the south-east near the centre of the grid (Fig. 5b).
Figure 5. Contour maps of the POC dry weight% contents of the mud fractions in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Seven months later (April 1996), i.e. after the severe ice winter, the same basic pattern in POC enrichment was still evident (Fig. 5c). Thus, a SW-NE corridor of lower POC contents (1.5-2.0%) was juxtaposed between higher values (3.0--4.0%) to the north-west and south-east. However, by the end of the following summer (October 1996) the pattern had largely disappeared, most of the 90 grid sites having POC enrichment levels of 2.5-3.0% except for higher values of 3.0-3.5% at some western localities (Fig. 5d). In the spring of both 1995 (March) and 1996 (April), the POC concentrations for the mud fractions varied from <50 to 150 g m 2 in the upper 5 cm of the sediments in the grid area as a whole, the higher values of 100-150 g m -2 being largely restricted to a SW-NE-oriented corridor in both cases (Fig. 6a, c). A similar spatial pattern was documented towards the end of summer (September 1995, October 1996) in both years when POC concentrations had risen to 150-300 g m -2 in the corridor, values of <100 g m 2 being largely limited to the north-west and central south-east sectors of the grid (Fig. 6b, d). During the four study campaigns, the spatial distributions in the POC concentrations for the sand fractions showed patterns which were the reverse of those documented for the mud fractions. Compared to the NW and SE corners of the grid, where sand POC measured 60-70 g m -2 in the upper 5-cm sediment layer, values were generally lower (<60 g m -2) in the SW-NE-oriented corridor at all times (Fig. 7). Over the summer of 1995 (March-September), a net import of POC was recorded in the sediments (mud+sand fractions) over much of the grid area, values varying in the range 20-80 g POC m -2 (Fig. 8a). A net export of <40 g POC m 2 w a s only recorded in a small central drainage area and 2 sites eastward thereof. This resulted in a positive POC budget of +262 t for this time interval, with nearly 44% (161 t) of the total POC flux (import+export) being associated with the sand fractions (Table 1). For the period spanning the severe winter (September 1995-April 1996), a negative POC budget of -477 t was documented for the grid area as a whole, the fluxes in sand POC (290 t) again making up over 40% of the total sediment budget (Table 1). Most grid localities experienced net exports of 40-160 g POC m -2 (Fig. 8b). Similarly to this severe winter but in contrast to the summer before, a net export of POC (-83 t) was observed over the summer of 1996 (April-October; Table 1). Once again, nearly 40% of the total POC flux (595 t) was linked to the sand fractions. The sites which experienced the highest export (80-160 g POC m ~ w e r e situated largely in the eastern grid sector (Fig. 8c). Compared to the 1995 summer period when a total flux of nearly 203 t POC was recorded for the mud fractions in the area as a whole, values were nearly twice as high (393 t POC) the following winter, a trend which continued (albeit less markedly) the summer thereafter (359 t POC; Table 1).
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Figure 6. Contour maps of the mud POC concentrations in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Figure 7. Contour maps of the sand POC concentrations in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Figure 8. Contour maps showing the gains and losses in the sediment (mud+sand) POC masses in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate gains (import), and negative values indicate losses (export) of sediment POC per unit area in a given time interval (g m -2 seasonS).
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Table 1. Mass budgets for the seasonal fluxes of mud, sand, and POC in the survey area on the Swinnplate tidal flat in the time intervals March-September 1995, September 1995-April 1996, and April--October 1996 (positive values are gains, and negative values are losses for the survey area in a given time interval, in tonnes*103 or tonnes dry mass per 0.7 km 2 per season; sediment = mud+sand; data extracted partly from Bartholom/i et al. 2000, this volume).
Sand mass (t*lO 3) Mud mass (t*lO 3) Sediment mass (t*lO 3) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t) Sand mass (t*lO 3) Mud mass (t*lO 3) Sediment mass (t*lO 3) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t) Sand mass (t*lO ~) Mud mass (t*lO 3) Sediment mass (t* 103) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t)
March 95September 95September 95 April 96 Export Import Export Import 28 119 227 47 1 9 18 2 29 128 245 49 30 131 240 50 21 182 340 53 51 313 580 103 Total fluxes (import+export) 147 274 10 20 157 294 161 290 203 393 364 683 Net fluxes (import-export) +91 -180 +8 -16 +99 -196 +101 -190 +161 -287 +262 -477
April 96October 96 Export Import 160 63 7 7 167 70 169 67 170 189 339 256 223 14 237 236 359 595 -97 0 -97 -102 +19 -83
4. D I S C U S S I O N A N D I M P L I C A T I O N S
In the East Frisian Wadden Sea, assessments of the organic matter (OM) content of sand (based on measurements of loss-on-ignition at 450~ yielded a mean value of 0.24 wt% OM (Delafontaine et al. 1996). The value of ca. 0.11 wt% POC recorded in the present study implies that the commonly used conversion factor of 1.8 (e.g., Trask 1939; Morgans 1956; Birch 1977) would underestimate OM enrichment by about 18% in this case. Rather, a conversion factor of 2.3 would be more appropriate for sands in the back-barrier tidal flats of the southern North Sea. In the m u d fractions on the Swinnplate, the seasonal assessments of POC enrichment carried out in the present study for the period March 1995-October 1996
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showed levels consistent with those documented by Delafontaine et al. (1996) in the same study grid in July/September 1994 (ca. 2-3 wt% POC). In the vicinity of M. edulis banks, therefore, mud organic carbon enrichment is generally about 20 times that recorded in sand. Spatial patterns in mud organic enrichment were not always homogenous in the study grid, lower values of 1.5-2.5 wt% POC having sometimes been recorded in a SW-NE corridor, whereas POC contents reached 4 wt% at some of the other sites. A 7-year 'mussel-watch' campaign, carried out from 1990 to 1996 on the Swinnplate, demonstrated that the mussels were also largely confined to this corridor, and mud accumulation was higher here than at other grid sites (cf. 10-30 wt% and <10 wt% mud, respectively; Bartholom~i et al. 2000, this volume). In other words, muds had higher levels of organic enrichment at sandier sites at some distance from the mussel banks, the reverse being the case at muddier sites close by. These large-scale patterns persisted despite substantial erosion of the tidal flats and decimation of the banks at the time of ice coverage early in 1996 (see below). These findings are consistent with 1) the basin-wide occurrence of increased OM enrichment documented for finegrained sediments on sandier tidal flats behind the islands of Spiekeroog, Langeoog and Baltrum, probably reflecting relatively higher clay and fine-silt contents in the muds at more exposed localities (Xu 2000; Delafontaine et al. 2000), and 2) the preservation of biogenic mud 'signatures' by burial (Bartholom~i et al. 2000, this volume). They also argue against the widespread assumption that biodeposits as such are particularly enriched in organic matter (e.g., Villbrandt et al. 1999). The value of 1.5-2.5 wt% POC documented in the biogenic muds integrates different pools of fine particles, each probably having a distinct POC signal. Thus, fine particles transported in suspension are trapped in the mussel banks because the 3-D structure of the banks increases surface roughness. In addition, the mussels produce faeces and pseudofaeces. Despite the fact that the psedofaeces possibly contain undigested algal cells, they have high contents of inorganic material (Clausen & Riisg~rd 1996; Jorgensen 1996). Compared to the faeces, therefore, OM enrichment is markedly lower in the pseudofaeces of many filter feeders (e.g., Cerastoderma edule; Navarro & Widdows 1997). However, it is well known that the production of pseudofaeces far outweighs that of faeces in turbid coastal environments (e.g., Navarro & Widdows 1997), and also in the Dutch Wadden Sea (Dankers et al. 1989). Such comparative data are lacking for M. edulis feeding under natural conditions in the German Wadden Sea. For example, Dittmann (1987) measured only total C (i.e. organic and inorganic C) in faeces. Nevertheless, it is evident that strong 'contamination' from, amongst other things, pseudofaeces has to be accounted for (at least conceptually) when attempting to link levels of, for example, degradation products in bulk sediments to the digestive activity of the mussels, an aspect which has been overlooked in other studies dealing with the Swinnplate M. edulis community (e.g., Villbrandt et al. 1997). Indeed, the results of the present study provide quantitative assessments of another important source of "contamination" in such studies, namely the sand component of the sediments.
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For the periods March-September 1995, September 1995-April 1996, and AprilOctober 1996, net fluxes (import-export) of sand POC constituted at least 40% of those for the sand and mud fractions combined. Furthermore, total fluxes (import +export) of sand POC made up nearly half (ca. 40%) of the total sediment fluxes at all times. Seeing that OM enrichment was comparatively low in the sands (see above), these high values are better explained by the large amounts of sand transported across the tidal flats, presumably as bed load and/or intermittent bottom suspension as a result of wave action during common autumn and winter storms (e.g., Fach 1996). Thus, net fluxes of sand always outweighed those of mud by an order of magnitude, and total fluxes of mud constituted only ca. 6-7% of the total sediment fluxes at all times in the Swinnplate grid area (Table 1; Bartholom~i et al. 2000, this volume). The admixture of substantial amounts of sand to the biogenic muds can lead to a dilution (decrease in concentration) of the OM pool in the sediments. For sand contents varying from ca. 80 wt% in muddier sediments in the mussel banks to >95 wt% on the adjoining sand flat (Bartholom~i et al. 2000, this volume), the POC concentrations decreased from ca. 300 to <100 g m -2 in the upper 5 cm of the sediments. In the Wadden Sea, therefore, sand reworking probably decreases the efficiency of material transfer to benthic organisms via biodeposition because consumers would have to forage in larger volumes of sediment for a given amount of food in this case. Depending on the relative proportions of sand and mud, however, Flemming and Delafontaine (2000) have demonstrated that, contrary to widespread opinion (for example, cf. Tyson 1995), sand admixture does not automatically dilute but can also condense fine particles in sediments. In this context, we contend that some views about the significance of sediment mixing in the feeding ecology of deposit feeders in mussel banks and, for that matter, in tidal-flat environments in general need to be revised (cf., for example, Snelgrove & Butman 1994). In the Swinnplate study grid, total fluxes of mud POC always made up over half (ca. 60%) of the mud and sand fluxes combined, also in the aftermath of the severe 1995-1996 winter when prolonged ice coverage resulted in the large-scale decimation of the mussel banks (Bartholom~i et al. 2000, this volume). Furthermore, in the summer of 1996 total fluxes of mud POC outweighed those documented the previous summer by a factor of nearly 2, despite mussel coverage having decreased by nearly 50% over this time interval. By implication, the cycling of organic matter in the fine fraction of sandy tidal flats could involve considerable amounts of buried material, particularly when ice scour and ensuing erosion excavate the remains of former mussel bank muds. Indeed, marked erosion reaching 0.7 m at places was recorded at the study site at the time (Bartholom~i et al. 2000, this volume). Independent evidence of facies structure with depth, gained by taking box cores at spot localities at the Swinnplate study site (Hertweck & Liebezeit 1996), also corroborate the importance of excavated mud horizons in the cycling of OM on Wadden Sea tidal flats. We conclude that, under the strong hydrodynamic conditions prevailing in the Wadden Sea, the turnover of sedimentary organic matter in intertidal M. edulis banks evidently involves larger amounts of sand than is commonly assumed. Frequent
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storm events and episodic ice coverage result in a substantial reworking of sandy sediments, thereby also promoting the admixture of OM sequestered in buried biogenic mud reservoirs. These results demonstrate the hitherto little recognised importance of also considering the effects of elevation changes and sediment mass physical properties when assessing and interpreting material fluxes in such highly dynamic environments as the Wadden Sea.
ACKNOWLEDGEMENTS
Our hearty thanks go to the captain, motorboat driver and crew of the research vessel Senckenberg for their expertise and unfailing good spirits during the fieldwork, oft carried out under difficult weather conditions. We also acknowledge the laboratory assistance of A. Rascke and numerous students. The work was sponsored partly by the Senckenbergische Naturforschende Gesellschaft, Frankfurt, and partly by the Federal Ministry of Education and Research, Bonn (Grant No. 03F0112A).
REFERENCES
Anonymous (1996) L~ingster Eiswinter seit 33 Jahren. Deutsche Seeschiffahrt 5/1996: 16. Bartholom~i, A., Flemming, B.W. & Delafontaine, M.T. (2000) Mass balancing the seasonal turnover of mud and sand in the vicinity of an intertidal mussel bank in the Wadden Sea (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Birch, G.F. (1977) Surficial sediments on the continental margin off the west coast of South Africa. Mar. Geol. 23: 305-337. Clausen, I. & Riisgard, H.U. (1996) Growth, filtration and respiration in the mussel Mytilus edulis: no evidence for physiological regulation of the filter-pump to nutritional needs. Mar. Ecol. Prog. Ser. 141: 37-45. Cressie, N.A.C. (1991) Statistics for Spatial Data. John Wiley & Sons, New York, 900 p. Dame, R.F. (ed.) (1993) Bivalve Filter Feeders in Estuarine and Coastal Ecosystem Processes. Springer, Berlin, NATO ASI Ser. 33. Dankers, N. & Koelemaij, K. (1989) Variations in the mussel population of the Dutch Wadden Sea in relation to monitoring of other ecological parameters. Helgol. Meeresunters. 43: 529-535. Dankers, N. Koelemaij, K. & Zegers, J. (1989) De rol van de mossel en de mosselcultuur in het ecosysteem van de Waddenzee. RIN Rep. 89/9, 66 p. Delafontaine, M.T., Bartholom~i, A., Flemming, B.W. & Kurmis, R. (1996) Volumespecific dry POC mass in surficial intertidal sediments: a comparison between biogenic muds and adjacent sand flats. Senckenbergiana marit. 26 (3/6): 167-178.
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Delafontaine, M.T., Flemming, B.W. & Kr6gel, F. (2000) Organic enrichment in backbarrier sediments of the Wadden Sea: a five-year environmental impact study spanning the Europipe landfall. In: Delafontaine, M.T., Flemming, B.W. & Vollmer, M. (eds), Environmental Impacts of Europipe. J. Coast. Res. Spec. Issue 27 (in press). Dittmann, S. (1987) Die Bedeutung der Biodeposite fiir die Benthosgemeinschaft der Wattsedimente, unter besonderer Beriicksichtigung der Miesmuschel Mytilus edulis (L.). Dissertation, Universit/it G6ttingen, 182 p. Fach, B. (1996) Die Entwicklung der Windverh~iltnisse in der Deutschen Bucht seit 1965 als wichtiger UmwelteinfluB auf die Kiistenzone. Diplom, Fachhochschule Wilhelmshaven (Germany), 75 p. Flemming, B.W. & Delafontaine, M.T. (1994) Biodeposition in a juvenile mussel bed of the East Frisian Wadden Sea (southern North Sea). Neth. J. Aquat. Ecol. 28: 289-297. Flemming, B.W. & Delafontaine, M.T. (2000) Mass physical properties of muddy intertidal sediments: some applications, misapplications and non-applications. Cont. Shelf Res. (in press). Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth J. Aquat. Ecol. 28: 299-307. Flemming, B.W. & Ziegler, K. (1995) High-resolution grain size distribution patterns and textural trends in the backbarrier environment of Spiekeroog Island (southern North Sea). Senckenbergiana marit. 26: 1-24. Graf, G. (1989) Benthic-pelagic coupling in a deep-sea benthic community. Nature 341: 437-439. Giinther, C.-P. & Niesel, V. (1999) Effects of the ice winter 1995/96. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 193-205. Hargrave, B.T. (1980) Factors affecting the flux of organic matter to sediments in a marine bay. In: Tenore, K.R. & Coull, B.C. (eds), Marine Benthic Dynamics. University South Carolina Press, Columbia, pp. 243-263. Hertweck, G. & Liebezeit, G. (1996) Biogenic and geochemical properties of intertidal biosedimentary deposits related to Mytilus beds. Proc. 29th EMBS, Vienna, August 1994. Mar. Ecol. (PSZNI) 17: 131-144. Jorgensen, C.B. (1996) Bivalve filter feeding revisited. Mar. Ecol. Prog. Ser. 142: 287-302. Krumbein, W.C. & Pettijohn, F.J. (1938) Manual of Sedimentary Petrography. D. Appleton-Century, New York, 549 p. Kurmis, R. (1995) Quart~irgeologische Detailkartierung der Swinnplate im Spiekerooger Rfickseitenwatt, siidliche Nordsee. Diplom, Universit~it Bremen, 98 p. Lewis, D.W. & McConchie, D. (1994) Analytical Sedimentology. Chapman & Hall, London, 197 p. Morgans, J.F.C. (1956) Notes on the analysis of shallow water soft substrates. J. Anim. Ecol. 25: 367-387.
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Navarro, J.M. & Widdows, J. (1997) Feeding physiology of Cerastoderma edule in response to a wide range of seston concentrations. Mar. Ecol. Prog. Ser. 152: 175-186. Nehls, G. & Thiel, M. (1993) Large-scale distribution patterns of the mussel Mytilus edulis in the Wadden sea of Schleswig-Holstein: do storms structure the ecosystem? Neth. J. Sea Res. 31(2): 181-187. Oost, A.P. (1995) Dynamics and sedimentary development of the Dutch Wadden Sea with emphasis on the Frisian inlet. Doctoral thesis, Universiteit Utrecht, Geol. Ultra. 126, 455 p. Smaal, A.C., Verhagen, J.H.G., Coosen, J. & Haas, H.A. (1986) Interaction between seston quantity and quality and benthic suspension feeders in the Oosterschelde, The Netherlands. Ophelia 26: 385-399. Snelgrove, P.V.R. & Butman, C.A. (1994) Animal-sediment relationships revisited: cause versus effect. Oceanogr. Mar. Biol. Ann. Rev. 32: 111-177. Tyson, R.V. (1995) Sedimentary Organic Matter. Chapman & Hall, London, 615 p. Ten Brinke, W.B.M., Augustinus, P.G.E.F. & Berger, G.W. (1995) Fine-grained sediment deposition on mussel beds in the Oosterschelde (The Netherlands), determined from echosoundings, radio-isotopes and biodeposition field experiments. Estuar. Coast. Shelf Sci. 40: 195-217. Trask, P.D. (1939) Organic content of recent marine sediments. In: Trask, P.D. (ed.) Recent Marine Sediments. Dover, New York, pp. 428-453. Villbrandt, M., Hild, A. & Dittmann, S. (1999) Biogeochemical processes in tidal flat sediments and mutual interactions with macrobenthos. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 95-132. Xu, W. (2000) Mass physical sediment properties and trends in a Wadden Sea tidal basin. Berichte, Fachbereich Geowissenschaften, Univ. Bremen, No. 157, 127 p.
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Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea) M. T. Delafontaine,* B. W. Flemming and A. Bartholom/i
Senckenberg Institute, Schleusenstr. 39a, D-26382 Wilhelmshaven, Germany
ABSTRACT
From March 1995 to October 1996, seasonal export and import fluxes of POC in the mud and sand fractions of an intertidal sand flat (area: 1400,500 m - 0.7 km 2) were investigated in the vicinity of a series of M. edulis banks behind Spiekeroog Island (East Frisian Wadden Sea). In January/February 1996 the region experienced a severe ice winter, and together with typical a u t u m n / w i n t e r storms, the budget assessments presented in this paper are thus representative of major physical and biological disturbances common in the Wadden Sea. The contents and concentrations of mud in the sediments of the survey area varied considerably, being generally higher in late summer and lower at the end of winter, although almost the same basic pattern was retained over the 19 month survey period. The average POC content of the local sands was 0.106%, whereas that of the mud fractions varied in the range 1.5-4.0 wt%, lower values being associated with high mud contents, higher ones with low mud contents. Overall, POC concentrations in the mud fractions varied from <50-150 g m 2, although peak values of 300 g m 2 occurred locally. Between March and September 1995 a net import of 262 t POC was recorded. In the period September 1995 to April 1996, which included the severe ice winter, a net export of 477 t POC took place. In contrast to the previous year, the summer following the ice winter, i.e. the period from April to October 1996, was characterised by a net export of 83 t POC. In all cases, 40-44% of the total POC fluxes were associated with the sand fraction, in spite of the generally low POC contents of sand. This observation reveals an important and hitherto unrecognised role of sand in total POC fluxes in the Wadden Sea, a feature evidently linked to the large turnover of sand which contributed >90% to the total sediment flux. Contrary to widespread belief, sand does not necessarily dilute substances attached to the m u d fraction in sand-mud mixtures but can actually act as a concentration mechanism because it induces substantial changes in sediment bulk density. This feature will, amongst others, have to be taken into account in the feeding ecology of benthic marine organisms, deposit feeders in particular.
* Corresponding author: M.T. Delafontaine e-mail: [email protected]
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1. INTRODUCTION It has long been known that a significant proportion of the pelagic particulate organic pool can be channelled to the benthic realm, creating a tight coupling between these 2 compartments in both coastal and deep-sea ecosystems (e.g., Hargrave 1980; Graf 1989; Dame 1993). In many deep-water environments, hydrodynamic conditions are weak enough to facilitate the deposition of particles originating in the productive upper layers. Shallow coastal waters, by contrast, are generally more turbulent and, consequently, the bottom sediments are often impoverished in fine-grained material. On the back-barrier tidal flats of the Wadden Sea (southern North Sea), mud flats have generally disappeared as a result of land reclamation (Flemming & Nyandwi 1994). Because of this loss in accommodation space, the physically controlled deposition of fine particles is today largely restricted to a few remaining embayments which are sheltered enough. By implication, the exchange of particulate matter between the water column and the sediments has become increasingly dependent on biodeposition, i.e. the condensation by filtering organisms of suspended organic and inorganic material in pseudofaeces and faeces. In this respect, the mussel Mytilus edulis is arguably the most important species in the region (e.g., Smaal et al. 1986), its filtering activity resulting in the formation of characteristic 'mud halos' in otherwise sandy areas where hydrodynamic conditions usually preclude the deposition of fine particles (e.g., Flemming & Ziegler 1995). The extent to which biogenic muds can be used as, amongst other things, a food source by benthic organisms depends largely on the size and turnover rate of this mud reservoir. In the Wadden Sea, fine-grained sediments are easily resuspended under existing hydrodynamic energy conditions, particularly during the more stormy autumn and winter months from about September to March (e.g., Flemming & Delafontaine 1994; Ten Brinke et al. 1995; Bartholom~i et al. 2000, this volume). In addition, a substantial mixing of biogenic muds with coarser grained sediments occurs on these tidal flats (e.g., Oost 1995), and sediments in the mussel banks mostly contain less than about 20% mud on a dry weight basis (e.g., Delafontaine et al. 1996; Bartholom/i et al. 2000, this volume). For the East Frisian Wadden Sea, seasonal mass balances have, in fact, demonstrated that total fluxes of mud (i.e. import+export) generally constitute only about 6-7% of total sediment fluxes in M. edulis banks (Bartholom/i et al. 2000, this volume). Higher enrichment levels in fine-grained material would tend to counterbalance this apparent dominance of coarser material in, for example, the cycling of particulate organic carbon (POC). However, the findings of Delafontaine et al. (1996, 2000) imply that such POC budgets are neither simple nor straightforward for the region. Thus, monthly POC enrichment levels varied strongly in the mud fractions of sandy back-barrier tidal flats behind the islands of Baltrum and Langeoog in the East Frisian Wadden Sea. Furthermore, in the mud fractions of sandy sites devoid of mussels, POC contents were generally higher (3.5-5.5 wt%) than those recorded in the vicinity of M. edulis banks where values fluctuated in the range 1.5-3.0%. Mussel
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occurrence also undergoes strong seasonal and annual fluctuations in the Wadden Sea (e.g., Dankers & Koelemaij 1989; Nehls & Thiel 1993; Flemming & Delafontaine 1994). Depending on factors affecting mussel survival and/or establishment, sandy sediments would therefore contain variable propor-tions of differently enriched mud pools. From March 1995 to October 1996 we investigated large-scale (700~103 m 2) seasonal export and import fluxes of POC in the mud and sand fractions of an intertidal sand flat behind Spiekeroog Island (East Frisian Wadden Sea), choosing a site which incorporated a series of M. edulis banks. POC data were collected concurrently to those for topographic elevation and sediment composition (cf. Bartholom/i et al. 2000, this volume). Roughly in the middle of the study period, a prolonged spell of ice coverage (ca. 50 days in January-February 1996; Anonymous 1996; Giinther & Niesel 1999) resulted in extensive damage and displacement of the mussels as well as considerable erosion (Bartholom/i et al. 2000, this volume). Together with typical autumn/winter storms, these budget assessments are therefore representative of major physical and biological disturbances common in the region.
2. MATERIALS AND METHODS 2.1. Study area, study grid The fieldwork was carried out on the Swinnplate tidal flat in the rear of the barrier island of Spiekeroog in the East Frisian Wadden Sea (southern North Sea; Fig. 1). The site represents a typical sand flat (mud contents usually <2 dry weight%) dominated by the polychaete Arenicola marina. A mixed community of the mussel M. edulis and the tube-dwelling polychaete Lanice conchilega with associated biogenic mud accumulations was present at the time of study (Kurmis 1995; Delafontaine et al. 1996).
Figure 1. Locality map (dotted lines in inset indicate watersheds).
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A grid which remained fixed for the duration of the study period (March 1995October 1996) consisted of 90 sampling/measuring points at 100-m intervals spanning several mussel beds over a rectangular area of 1400,500 m (0.7 km2; Fig. 2). The positions of the 90 grid points were fixed by laser theodolite with an accuracy of
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Figure 2. Location of sampling grid (bold dots) on the Swinnplate sand flat in the rear of Spiekeroog Island (cf. Fig. 1). Depth contours are metres below German topographic chart datum.
2.2. Mass balancing Mass balances of export/import fluxes for mud, sand, and POC were determined for three consecutive seasons in the time intervals March-September 1995 (7 months, summer), September 1995-April 1996 (8 months, winter), and April-October 1996 (7 months, summer). 2.2.1. Terminology In the following we define some key terms as used in the present study. Seasonal: a time interval spanning winter or summer, and lasting 7-8 months. Sand: sediment size fractions 0.063-2 mm. Mud: sediment size fractions <0.063 mm. Sediment: unfractionated sand+mud. Dry bulk density: dry mass of sediment (desalinated) in a unit volume of water-saturated sediment. Content of a substance in the sediment: dry mass of the substance in a unit mass of water-saturated sediment. Concentration of a substance in the sediment: dry mass of the substance (desalinated) in a unit volume of water-saturated sediment. Dry weight% content of a substance in the sediment: (dry mass of the substance/dry mass sediment)*100. Export: flux from the study area (loss), reflecting net erosion in a given time interval. Import: flux to the study area (gain), reflecting net deposition in a given time interval. Total flux: export+import. Mass budgets (net fluxes): import mass-export mass. Flux units" tonnes dry sediment (sand, mud) or POC per unit area per season. Tonne or t: metric ton (= 1000 kg).
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2.2.2. Procedure
Topography was measured by means of precision levelling at all 90 grid points, using a laser theodolite (see above) to determine elevations to ca. 1 cm relative to the German topographic chart datum (NN = normal null). Measurements were carried out every 7-8 months in the period March 1995-October 1996 (Bartholom~i et al. 2000, this volume). Export/import fluxes of sand and mud were determined by calculating the differences between the topographic elevations recorded at the beginning and end of a given 7 to 8-month study interval, and then generating corresponding changes in wet sediment volume (Fig. 3). These data were used in combination with concurrent measurements of sediment composition (mud content), and a site-specific relationship between dry bulk density and mud content (see below) to convert volumetric changes into gains or losses of sand and mud masses (Bartholom~i et al. 2000, this volume). Gains and losses in sand masses were multiplied by the POC content of local sands in order to calculate corresponding changes in sand POC masses. In order to calculate the mud POC masses, gains and losses in mud masses were coupled with the site-specific mud POC contents measured at the end and at the beginning of each study interval, respectively. to determine BULK SEDIMENT FLUX erosion = loss = export deposition = gain = import
measure VOLUME CHANGE by repeated precision levelling by applying site-specific DRY BULK DENSITY, convert into BULK SEDIMENT MASS (gain or loss)
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Figure 3. Flow diagram of the procedure followed to measure export/import fluxes of mud, sand, and POC on the Swinnplate sand flat.
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2.2.3. Sediments Concurrently to the elevation measurements, surficial sediments were collected to depths of up to 5 cm at all 90 grid points, at least 5 subsamples being pooled in each case in order to reduce the probable sampling error by at least 50% (e.g., Krumbein & Pettijohn 1938). The samples were desalinated in the laboratory, and then wet sieved through a 0.063-mm mesh. The resulting sand and m u d fractions were oven dried at 65~ for 24 hours, weighed, and these values were then used to calculate the m u d contents of the sediments (e.g., Lewis & McConchie 1994). The empirical relationship between the dry bulk density and m u d content of the sediments used in the present study was extracted from Flemming & Delafontaine (2000), i.e. y = -0.7955892+2.3863045e ~-~1~5829277~where y - dry bulk density of the surficial sediment (upper 5 cm; mass (g) of desalinated dry sediment per cm 3 of water-saturated sediment), x = weight% of dry m u d content of the sediment, n = 337, r = -0.9847. 2.2.4. POC POC contents were determined in all mud fractions (mud POC content = [dry mass POC in m u d / d r y mass mud] * 100). Aliquots were ground by means of a pestle and mortar and, after acid leaching with HC1 fumes for 15-24 hours, the organic carbon was measured by means of an Hereaus CHNS analyser in each case (standard: acetanilide; reproducibility: +4.8%, n = 10; cf. Delafontaine et al. 1996). POC contents of whole (unfractionated) sediments were measured in additional samples collected on the Swinnplate and in nearby Jade Bay. In this case each sample was halved before desalting, resulting in 2 groups of complementary subsamples. One group served for the POC analyses after grounding in a mechanical mill, the other group for the determination of mud contents. This data set was used to estimate the mean POC content of local sands by means of regression analysis. 2.4. Data plotting Contour plots were computer generated by means of the software package SURFER (Windows Golden Software Inc. RT) using the distance interpolation procedure (e.g., Cressie 1991).
3. RESULTS
The variability of mud concentrations (i.e. volume-specific m u d masses) in the survey area are illustrated in Fig. 4. It is clearly evident that almost the same basic pattern was retained over the 19 month survey period, in spite of the fact that m u d concentrations varied considerably, being generally higher in late summer and lower at the end of winter. The similarity of the pattern was all the more surprising as there was also considerable variation in sediment turnover as documented by substantial local elevation changes (cf. Bartholom~i et al., this volume). Mud is thus either
Turnover of POC in mud and sand
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worked into the sediment to depths of up to several decimeters, or older m u d banks essentially occupying the same area are exhumed in the course of erosion.
Figure 4. Mud concentrations (volume-specific m u d masses) in the survey area over a period of 19 months (a: March 1195; b: September 1995; c: April 1996; and d: October 1996). Note the strong variability from survey to survey.
Regression analysis of POC contents in sediments of varying sand-mud proportions (ca. 0.5-65 wt% mud) shows that the POC content of local sands is 0.106%. Thus, y - 0.106+0.023x where y - POC content of sediment, x - m u d content of sediment, n - 55, r - 0.86.
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In spring (March) 1995, the POC contents of the mud fractions varied in the range 1.5-2.0 wt% in large areas of the Swinnplate grid (Fig. 5a). Higher values of 2.0-3.0% POC were recorded in the north-western and more centrally situated southern sectors. Contents had generally risen to 1.5-2.5% by the end of summer (September) that year, and higher values of 3.0-4.0% were again found at the north-west sites as well as more to the south-east near the centre of the grid (Fig. 5b).
Figure 5. Contour maps of the POC dry weight% contents of the mud fractions in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Seven months later (April 1996), i.e. after the severe ice winter, the same basic pattern in POC enrichment was still evident (Fig. 5c). Thus, a SW-NE corridor of lower POC contents (1.5-2.0%) was juxtaposed between higher values (3.0--4.0%) to the north-west and south-east. However, by the end of the following summer (October 1996) the pattern had largely disappeared, most of the 90 grid sites having POC enrichment levels of 2.5-3.0% except for higher values of 3.0-3.5% at some western localities (Fig. 5d). In the spring of both 1995 (March) and 1996 (April), the POC concentrations for the mud fractions varied from <50 to 150 g m 2 in the upper 5 cm of the sediments in the grid area as a whole, the higher values of 100-150 g m -2 being largely restricted to a SW-NE-oriented corridor in both cases (Fig. 6a, c). A similar spatial pattern was documented towards the end of summer (September 1995, October 1996) in both years when POC concentrations had risen to 150-300 g m -2 in the corridor, values of <100 g m 2 being largely limited to the north-west and central south-east sectors of the grid (Fig. 6b, d). During the four study campaigns, the spatial distributions in the POC concentrations for the sand fractions showed patterns which were the reverse of those documented for the mud fractions. Compared to the NW and SE corners of the grid, where sand POC measured 60-70 g m -2 in the upper 5-cm sediment layer, values were generally lower (<60 g m -2) in the SW-NE-oriented corridor at all times (Fig. 7). Over the summer of 1995 (March-September), a net import of POC was recorded in the sediments (mud+sand fractions) over much of the grid area, values varying in the range 20-80 g POC m -2 (Fig. 8a). A net export of <40 g POC m 2 w a s only recorded in a small central drainage area and 2 sites eastward thereof. This resulted in a positive POC budget of +262 t for this time interval, with nearly 44% (161 t) of the total POC flux (import+export) being associated with the sand fractions (Table 1). For the period spanning the severe winter (September 1995-April 1996), a negative POC budget of -477 t was documented for the grid area as a whole, the fluxes in sand POC (290 t) again making up over 40% of the total sediment budget (Table 1). Most grid localities experienced net exports of 40-160 g POC m -2 (Fig. 8b). Similarly to this severe winter but in contrast to the summer before, a net export of POC (-83 t) was observed over the summer of 1996 (April-October; Table 1). Once again, nearly 40% of the total POC flux (595 t) was linked to the sand fractions. The sites which experienced the highest export (80-160 g POC m ~ w e r e situated largely in the eastern grid sector (Fig. 8c). Compared to the 1995 summer period when a total flux of nearly 203 t POC was recorded for the mud fractions in the area as a whole, values were nearly twice as high (393 t POC) the following winter, a trend which continued (albeit less markedly) the summer thereafter (359 t POC; Table 1).
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Figure 6. Contour maps of the mud POC concentrations in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Figure 7. Contour maps of the sand POC concentrations in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Figure 8. Contour maps showing the gains and losses in the sediment (mud+sand) POC masses in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate gains (import), and negative values indicate losses (export) of sediment POC per unit area in a given time interval (g m -2 seasonS).
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Table 1. Mass budgets for the seasonal fluxes of mud, sand, and POC in the survey area on the Swinnplate tidal flat in the time intervals March-September 1995, September 1995-April 1996, and April--October 1996 (positive values are gains, and negative values are losses for the survey area in a given time interval, in tonnes*103 or tonnes dry mass per 0.7 km 2 per season; sediment = mud+sand; data extracted partly from Bartholom/i et al. 2000, this volume).
Sand mass (t*lO 3) Mud mass (t*lO 3) Sediment mass (t*lO 3) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t) Sand mass (t*lO 3) Mud mass (t*lO 3) Sediment mass (t*lO 3) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t) Sand mass (t*lO ~) Mud mass (t*lO 3) Sediment mass (t* 103) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t)
March 95September 95September 95 April 96 Export Import Export Import 28 119 227 47 1 9 18 2 29 128 245 49 30 131 240 50 21 182 340 53 51 313 580 103 Total fluxes (import+export) 147 274 10 20 157 294 161 290 203 393 364 683 Net fluxes (import-export) +91 -180 +8 -16 +99 -196 +101 -190 +161 -287 +262 -477
April 96October 96 Export Import 160 63 7 7 167 70 169 67 170 189 339 256 223 14 237 236 359 595 -97 0 -97 -102 +19 -83
4. D I S C U S S I O N A N D I M P L I C A T I O N S
In the East Frisian Wadden Sea, assessments of the organic matter (OM) content of sand (based on measurements of loss-on-ignition at 450~ yielded a mean value of 0.24 wt% OM (Delafontaine et al. 1996). The value of ca. 0.11 wt% POC recorded in the present study implies that the commonly used conversion factor of 1.8 (e.g., Trask 1939; Morgans 1956; Birch 1977) would underestimate OM enrichment by about 18% in this case. Rather, a conversion factor of 2.3 would be more appropriate for sands in the back-barrier tidal flats of the southern North Sea. In the m u d fractions on the Swinnplate, the seasonal assessments of POC enrichment carried out in the present study for the period March 1995-October 1996
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showed levels consistent with those documented by Delafontaine et al. (1996) in the same study grid in July/September 1994 (ca. 2-3 wt% POC). In the vicinity of M. edulis banks, therefore, mud organic carbon enrichment is generally about 20 times that recorded in sand. Spatial patterns in mud organic enrichment were not always homogenous in the study grid, lower values of 1.5-2.5 wt% POC having sometimes been recorded in a SW-NE corridor, whereas POC contents reached 4 wt% at some of the other sites. A 7-year 'mussel-watch' campaign, carried out from 1990 to 1996 on the Swinnplate, demonstrated that the mussels were also largely confined to this corridor, and mud accumulation was higher here than at other grid sites (cf. 10-30 wt% and <10 wt% mud, respectively; Bartholom~i et al. 2000, this volume). In other words, muds had higher levels of organic enrichment at sandier sites at some distance from the mussel banks, the reverse being the case at muddier sites close by. These large-scale patterns persisted despite substantial erosion of the tidal flats and decimation of the banks at the time of ice coverage early in 1996 (see below). These findings are consistent with 1) the basin-wide occurrence of increased OM enrichment documented for finegrained sediments on sandier tidal flats behind the islands of Spiekeroog, Langeoog and Baltrum, probably reflecting relatively higher clay and fine-silt contents in the muds at more exposed localities (Xu 2000; Delafontaine et al. 2000), and 2) the preservation of biogenic mud 'signatures' by burial (Bartholom~i et al. 2000, this volume). They also argue against the widespread assumption that biodeposits as such are particularly enriched in organic matter (e.g., Villbrandt et al. 1999). The value of 1.5-2.5 wt% POC documented in the biogenic muds integrates different pools of fine particles, each probably having a distinct POC signal. Thus, fine particles transported in suspension are trapped in the mussel banks because the 3-D structure of the banks increases surface roughness. In addition, the mussels produce faeces and pseudofaeces. Despite the fact that the psedofaeces possibly contain undigested algal cells, they have high contents of inorganic material (Clausen & Riisg~rd 1996; Jorgensen 1996). Compared to the faeces, therefore, OM enrichment is markedly lower in the pseudofaeces of many filter feeders (e.g., Cerastoderma edule; Navarro & Widdows 1997). However, it is well known that the production of pseudofaeces far outweighs that of faeces in turbid coastal environments (e.g., Navarro & Widdows 1997), and also in the Dutch Wadden Sea (Dankers et al. 1989). Such comparative data are lacking for M. edulis feeding under natural conditions in the German Wadden Sea. For example, Dittmann (1987) measured only total C (i.e. organic and inorganic C) in faeces. Nevertheless, it is evident that strong 'contamination' from, amongst other things, pseudofaeces has to be accounted for (at least conceptually) when attempting to link levels of, for example, degradation products in bulk sediments to the digestive activity of the mussels, an aspect which has been overlooked in other studies dealing with the Swinnplate M. edulis community (e.g., Villbrandt et al. 1997). Indeed, the results of the present study provide quantitative assessments of another important source of "contamination" in such studies, namely the sand component of the sediments.
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For the periods March-September 1995, September 1995-April 1996, and AprilOctober 1996, net fluxes (import-export) of sand POC constituted at least 40% of those for the sand and mud fractions combined. Furthermore, total fluxes (import +export) of sand POC made up nearly half (ca. 40%) of the total sediment fluxes at all times. Seeing that OM enrichment was comparatively low in the sands (see above), these high values are better explained by the large amounts of sand transported across the tidal flats, presumably as bed load and/or intermittent bottom suspension as a result of wave action during common autumn and winter storms (e.g., Fach 1996). Thus, net fluxes of sand always outweighed those of mud by an order of magnitude, and total fluxes of mud constituted only ca. 6-7% of the total sediment fluxes at all times in the Swinnplate grid area (Table 1; Bartholom~i et al. 2000, this volume). The admixture of substantial amounts of sand to the biogenic muds can lead to a dilution (decrease in concentration) of the OM pool in the sediments. For sand contents varying from ca. 80 wt% in muddier sediments in the mussel banks to >95 wt% on the adjoining sand flat (Bartholom~i et al. 2000, this volume), the POC concentrations decreased from ca. 300 to <100 g m -2 in the upper 5 cm of the sediments. In the Wadden Sea, therefore, sand reworking probably decreases the efficiency of material transfer to benthic organisms via biodeposition because consumers would have to forage in larger volumes of sediment for a given amount of food in this case. Depending on the relative proportions of sand and mud, however, Flemming and Delafontaine (2000) have demonstrated that, contrary to widespread opinion (for example, cf. Tyson 1995), sand admixture does not automatically dilute but can also condense fine particles in sediments. In this context, we contend that some views about the significance of sediment mixing in the feeding ecology of deposit feeders in mussel banks and, for that matter, in tidal-flat environments in general need to be revised (cf., for example, Snelgrove & Butman 1994). In the Swinnplate study grid, total fluxes of mud POC always made up over half (ca. 60%) of the mud and sand fluxes combined, also in the aftermath of the severe 1995-1996 winter when prolonged ice coverage resulted in the large-scale decimation of the mussel banks (Bartholom~i et al. 2000, this volume). Furthermore, in the summer of 1996 total fluxes of mud POC outweighed those documented the previous summer by a factor of nearly 2, despite mussel coverage having decreased by nearly 50% over this time interval. By implication, the cycling of organic matter in the fine fraction of sandy tidal flats could involve considerable amounts of buried material, particularly when ice scour and ensuing erosion excavate the remains of former mussel bank muds. Indeed, marked erosion reaching 0.7 m at places was recorded at the study site at the time (Bartholom~i et al. 2000, this volume). Independent evidence of facies structure with depth, gained by taking box cores at spot localities at the Swinnplate study site (Hertweck & Liebezeit 1996), also corroborate the importance of excavated mud horizons in the cycling of OM on Wadden Sea tidal flats. We conclude that, under the strong hydrodynamic conditions prevailing in the Wadden Sea, the turnover of sedimentary organic matter in intertidal M. edulis banks evidently involves larger amounts of sand than is commonly assumed. Frequent
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storm events and episodic ice coverage result in a substantial reworking of sandy sediments, thereby also promoting the admixture of OM sequestered in buried biogenic mud reservoirs. These results demonstrate the hitherto little recognised importance of also considering the effects of elevation changes and sediment mass physical properties when assessing and interpreting material fluxes in such highly dynamic environments as the Wadden Sea.
ACKNOWLEDGEMENTS
Our hearty thanks go to the captain, motorboat driver and crew of the research vessel Senckenberg for their expertise and unfailing good spirits during the fieldwork, oft carried out under difficult weather conditions. We also acknowledge the laboratory assistance of A. Rascke and numerous students. The work was sponsored partly by the Senckenbergische Naturforschende Gesellschaft, Frankfurt, and partly by the Federal Ministry of Education and Research, Bonn (Grant No. 03F0112A).
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