Small-scale patchiness of benthos and sediment parameters in a freshwater tidal mud-flat of the river Elbe Estuary (Germany)

Limnologica 30 (2000) 183-192 http://www.urbanfischer.de/j ournals/limno

LIMNOLOGICA © by Urban & Fischer Verlag

German Oceanographic Museum, Stralsund, Germany

Small-scale Patchiness of Benthos and Sediment Parameters in a Freshwater Tidal Mud-flat of the River Elbe Estuary (Germany)* GOTZ B. REINICKE With 8 Figures and 2 Tables Key words: Patchiness, benthic Cladocera, Chironomidae, Oligochaeta, Turbellaria, meiofauna, diatoms, chlorophyll

Abstract The freshwater tidal mud-fiat environments of the Elbe estuary are unique and fugitive systems with highly specific conditions. Fine grained sediments harbour a specialised and opportunistic benthic community, largely dominated by tubificid Oligochaeta and the benthic Cladocera Leydigia acanthocercoides and lIiocryptus sordidus (Crustacea, Phyllopoda) among macrofauna, together with Chironomidae. Turbellaria, Nematoda, Rotatoria, Gastrotricha and Tardigrada predominate the meiofauna. This study presents results of a small-scale sampling design, investigating distribution patterns of fauna in an area of 625 cm2 (25x25 cm). Patchy patterns of faunal distribution of Oligochaeta, Chironomidae, Cladocera, Turbellaria and meiofauna taxa (Nematoda, Tardigrada, Gastrotricha, Rotatoria) indicate intrinsic control of abundance on small scales of less than 10 cm within the sample area (e.g. prey - predator relationships). Benthic colonisation of sediments is concentrated in the uppermost lcm sediment layer. Parallel analysis of chloroplastic pigment contents (chlorophyll-a and chloroplastic pigment equivalents - CPE) revealed regular distribution of chlorophyll-a content values in the uppermost sediment centimetre layer, reflecting recent sedimentation of abundant Actinocyclus normanii (Diatomeae) from the estuarine plankton. In deeper sediment layers distribution of chloroplastic pigment contents was patchy, and showed an overall maximum in the 2"dcentimetre layer.

1. Introduction Studies of benthic freshwater and marine soft bottom communities cover a wide spectrum of spatial and temporal scales, ranging from on-the-spot analyses of specific community structures to extended long-term monitoring programs. Among the wide variety of benthic environments the highly specialised freshwater tidal flats within estuary systems, how* This paper is dedicated to Prof. Dr. HARTMUTKAUSCHon the occasion of his 60 thbirthday.

ever, so far only attracted limited attention, mainly due to their only local significance and their rather anecdotal relevance in larger ecological contexts of aquatic research project routines. An example of these rare environments are the shallow tidal flats of the lower freshwater region of the River Elbe in northern Germany (e.g. HA~GE & GREISER 1996). Diurnal tides entering the estuary from the North Sea induce diurnal water level changes of about 2-3 m in the port area of Hamburg, at about 120 km from the actual coastline and well beyond the brackish transition zone. KRIEG (1996) in investigations on the occurrence and distribution of subtidal benthic fauna identified the transition zone between brackish and freshwater on both sides of Luehesand, at km 648.5 in the lower Elbe. On shallow river banks outside the inland dykelines the changing water level has created unique freshwater tidal flat environments, consisting predominantly of fine grained sediments (mud-flats), which harbour specialised assemblages of freshwater benthic fauna and flora (e.g. PFANNKUCHE et al. 1975; PFANNKUCHE 1981; HAGGE 1985). Due to the tidal regime, the benthic freshwater organisms are regularly exposed to unfavourable conditions (sunlight, warming, direct rainfall, seasonal climate changes and intense sedimentation). Only short living, not much specialised "opportunistic" freshwater species with high reproduction rates colonize these environments. A small number of ecological studies covers selected aspects of freshwater flats along the River Elbe (especially the Mtihlenberger Loch and F~hrmannsander Watt: CASPERS 1948, 1984; HENNIG ~% ZANDER 1981; GRIMM 1968; KAFEMANN1992; POSEWANG-KONSTANTIN et al. 1992; MITSCHKE ~¢ GARTHE 1994; GRIMM ~: KIESEWETTEa 1996). Patchy distribution patterns of the benthic community of the freshwater tidal flat on the island "Nel3sand" (at km 635-640) yet had been investigated by VORBERG (1993), carried out synchronously with the present study. 0075-9511/00/30/02-183 $ 12.00/0

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the freshwater tidal region of the River Elbe (km 635-640; Fig. 1). At the eastern end of the island, a shallow embayment of ca. 350,000 m ~ opens towards the south into the Hahn0fer Nebenelbe. The central part of the bay comprises of approx. 200,000 m 2 of high intertidal m u d deposits of up to 1.5 m thickness overlying coarser, sandy sediment (Fig. 1). The bight represents one of very few remaining freshwater tidal environments at the River Elbe since the river bed was channelled and dredged to allow large seagoing vessels to reach the Hamburg port. Tidal range is about 2.8-2.9 m, with 1.5-1.8 m of water above the sediment surface at high tide, measured during this study.

Benthic community patterns are known to be patchy in space and time, and the study of factors controlling their dynamics have to include the complex relations of biotic elements with the abiotic environment. Representative sampling depends on the scale of investigation, on the problems to be studied and on specific features of the organisms involved, such as population sizes, abundance dynamics, seasonality etc. (e.g. ELHO~ 1977; V o ~ 1993). Small scale patchiness is undetectable from sampling programs designed for large-scale rapid evaluation of benthos dynamics, and environmental factors controlling patchy distribution differ depending on the scale of investigation. The minimal resolution of small-scale investigations, however, is generally limited by the amount of sampling material available for the analysis of various parameters, depending on the methodology applied. The study of micro-environments and their communities constituents on the scale level of less than 1 m has thus become a specialised field of benthos research. Small-scale distribution patterns of meiofauna were investigated e.g. by ARI.T (1973) on test areas of 40x40 cm, demonstrating benthos patchiness within centimetre ranges in a brackish water environment. Given the observation of patchiness in the benthic environment, the intention of the present study was to investigate some interrelations of environmental conditions and the organisms community on a small-scale area. The set of parameters studied (sediment features, distribution and abundance of fauna, distribution of chloroplastic pigments as an indicator of benthic primary production and food resources) is used to provide an initial small-scale characterisation of the microenvironmental conditions in a fresh water tidal mud flat.

3. Material and Methods Sampling The highest spot on the tidal mud flat sediment bank (i.e. falling dry first and getting flooded last during the low water period) was selected. 25 round brass corers were used to take samples of 1 cm sediment layers to a depth of 5 cm on a total sampling area of 625 cm 2 (25x25 cm). For the design of corers for sediment and benthic macrofauna samples see Fig 2. For sampling 15 corers were positioned in three close parallel rows (Fig. 3). After sampling the first row of cores the 5 empty corers were positioned as the 4th row, the 2nd row of corers became the 5th row. 25 cores (each covering 18.1 cm~) thus covered a total of 72.4% (452.4 cm 2) of the sampling area. For sampling of centimetre layers the corers were taken out of the sediment, a pistil was used to adjust the core surface with the 1~' slit, then, by sliding the spatula through the 2 "~ slit the upper corecentimetre got separated and was washed into a sampling container, using stained formalin solution (4%). A 2 nd spatula slid into the 3rd slit isolated the 2 "a core-centimetre, which was washed into the next container after removing the 1~ spatula, etc. For sediment analysis the top 5 cm of the cores were collected as one sample (station 1).

2. Study area The study area of the present investigation is situated on the artificial island "Ne6sand" about 20 k m west of Hamburg in N

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Sampling was carried out in July and September 1987. Two sets of samples were collected on each occasion after photographing the respective sampling areas. In July 1987 one set of 25 samples was taken for sediment analysis (station 1), and one for the analysis of small scale fauna distribution (station 2). In September 1987 one set of samples was collected for the analysis of small scale fauna distribution (station 3). Spaces between the brass corers for fauna samples were then used to overly a grid of 36 meio-corers to extract a set of 36 samples for the analysis of photosynthetic pigment contents (Fig. 3). Meio-corers were 5 ml medical syringes cut open at the tip, = 12 ram, ca. 1.13 cm 2, together covering an additional area of about 40.7 cm 2 (= 6.5% of the total sampling area). The set of 36 meio-cores were sampled as a whole and deep-frozen at -20 °C after 0.5 h of transportation, to be stored until further analysis.

Sampleprocessing Sediment 25 sediment cores of 5 cm were taken at station 1 (wet weight of samples: 100-120 g). For the analysis of different parameters subsamples were taken: ca. 40 g were dried (105 °C, 24 h) and combust-

ed (550 °C, lh) for the analysis of water and organic content, 10 g separated for Laser-analysis of the silt fraction (FrtiTSC~ 1987), and 50-70 g for particle size analysis (BUCHANAN1984). Samples for the Laser particle-size-analysis were treated with H202 to disintegrate particle agglomerations. Ultrasound was applied to produce homogenous sediment suspension for analysis.

Fauna Samples for the analysis of faunal distribution at station 2 had been stained using bengal-red with the preservative (formalin 4%). For the extraction of fauna from the sediments a flotation technique was applied, using LUDOX AM [p = 1.21 g cm 3, Du Pont de Nemours; after PFANNI(UCHE& TIJIEL (1988)]. Centrifugation at 1500 rpm for 10 rain was applied in order to separate diatoms from the faunal fraction in the supernatant. Samples were then counted quantitatively. Chlorophyll Frozen samples for chlorophyll analysis were cut into separate depth centimetre blocs, starting from the surface layer. Each bloc was ground in 8 ml acetone (90%), with a spatula tip of MgHCO3 (to eliminate pore water) and ca. 1 ml of small glass pearls (~3 0.5 ram, after GREISER 1988). After centrifugation for 30 rain at 4000 rpm and 0 °C content of chlorophyll-a and total chloroplastic pigment equivalents (CPE) were measured using a Turner fluorometer at 665 nm and analysed (after HOLM-HANSENet al. 1965; SOLTWEDEL1987). Statistical analysis For sediment parameters (water and organic content), abundance of macrofauna taxa (Oligochaeta, Cladocera, Chironomidae, Turbellaria) and pigment contents data (chlorophyll-a and CPE) the chisquare test was applied to detect patterns of dispersion, i.e. regular or contagious/"patchy" distribution (ELLIOT 1977). Regarding the disL i m n o l o g i c a 30 (2000) 2

185

tribution of fauna the Spearman ranking-correlation test was applied to test correlation of data sets (stations 2 and 3; SACHS 1982). To compare distribution of faunal taxa and chloroplastic pigment contents the mean of each four neighbouring pigment values was related to the respective fauna sample, also applying the Spearman rankingcorrelation test (station 3, September 1987).

4. Results Sediment Results of the analysis of sediment parameters are shown in Figs. 4a-d. Water content values ranged from 6 0 - 6 6 % of wet weight, organic components in sediments ranged from 8.4-9.3% of sediment dry weight in the test area. Results of

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grain size analysis demonstrated a high proportion of the poorly sorted silt-fraction (sorting coefficient So = 2.4), with a Median value of 28.6 g m (n = 25, SD = 1.9) calculated from the sieving analysis (Fig. 4c), and 25.9 p m (n = 25, SD = 1.8) calculated from the Laser particle-size-analysis. The difference in results of both methods is statistically significant at p < 0.001. Detailed results of the Laser particle-sizeanalysis showed no significant differences in sediment grain size composition between 25 samples from the test area (station 1).

Fauna The study presents results on most abundant benthic macrofauna taxa from the surface centimetre of the cores only

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Fig. 4a-d. Results of the analysis of sediment parameters in the test area (station 1) sampled in July 1987. a: Distribution of water content [% wet weight] - axes give sample coordinates; b: distribution of organic material content [% dry weight] - axes give sample coordinates; c, d: grainsize distributions from sieving (c) and Laser-analysis (d). 186

Limnologica 30 (2000) 2

(Figs 5a-d): Oligochaeta (Tubificidae largely predominant), Chironomidae, Turbellaria and two cladoceran species (Crustacea, Phyllopoda). September sample analysis was reduced to a comer centric radial pattern of 12 samples (Fig. 6). Meiofauna, such as Rotatoria, Gastrotricha, Nematoda, Tardigrada occurring in vast numbers were only counted in a series of four samples of the 1st row (A, B, C, E in July 1987) and one sample in September 1987 (Fig. 7).

Distribution patterns of macrofauna in July showed certain agreement with the small-scale morphology of the sediment surface and a remaining rest water puddle in the sample area of station 2. Total counts of Oligochaeta (Fig. 5a) showed the maximum number of individuals/sample within an elevated ridge of sediment crossing the central range of the sample area. Numbers decreased to about half the maximum counts with sloping sediment surface and overlying

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Fig. 5a-d. Distribution patterns of predominant macrofauna taxa in the uppermost sediment layer of the test area (station 2, size: 25×25 cm, 1st cm) sampled in July 1987 [Ind./sample, sample size = 18.1 cm2]. Axes give sample coordinates, shaded areas are not represented by data. a: Oligochaeta; b: Chironomidae (note: individual numbers counted maximum 6 in July and 21 in September); c: L. acanthocercoides; d: 1. sordidus.

Limnologica 30 (2000) 2

187

rest water. Similar patterns were observed in the distribution of the two cladoceran species, Leydigia acanthocercoides (FISCHER) and Iliocryptus sordidus (L[EvIN) (Fig. 5c, d), showing significant correlation (total Cladocera and L sordidus, p < 0.01) with the Oligochaeta distribution. In con-

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trast, the distribution of Chironomidae showed a minimum within the sediment ridge and was not correlated with the abundances of Oligochaeta (Fig. 5b; note: individual numbers/sample counted maximum 6 in July and 21 in September). Dispersion pattern of all taxa was contagious (chisquare test), except for the Chironomidae at station 2 in July. In September (Fig. 6) mean abundance of Oligochaeta had increased by 37% (970 Ind. dm-2), while total abundance of Cladocera had increased by factor 7 (L sordidus 470 Ind. dm -2, L. acanthocercoides 2150 Ind. din-2). Relative dominance of both species, however, reversed from 81% versus 18% in July to 18% versus 82% in September, for I. sordidus and L. acanthocercoides, respectively. Abundances of Chironomidae (50 Ind. dm -2) also increased, although total numbers of individuals remained relatively low. Different from July, Chironomidae individuals per sample showed a significant correlation with the abundance of Oligochaeta (p < 0.001). Turbellaria were observed in September only (83 Ind. din-z), reaching a mean abundance of 15 _+ 10.3 Ind./sample, showing patchy distribution in the topmost sediment cent•metre only (X2, p < 0.001). Individual counts correlated with Cladocera counts (p < 0.05). Abundance of meiofauna taxa was concentrated in the topmost sediment layer (1 cm), showing variation between samples in July (n = 4) and slight changes by September (n = 1): Most abundant Nematoda ranged from 5000-7000 Ind./sample, while Tardigrada reached 2000-3000 Ind./sample in July and decreased in September (for detailed results compare REINICKE 1988). For Rotatoria and Gastrotricha, both ranging from 400-500 Ind./sample in July, abundance decreased in Rotatoria and doubled in Gastrotricha in September. As in the macrofauna, abundance concentrated in the uppermost sediment cent•metre.

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Results of chlorophyll-a and CPE contents from the 1st sediment cent•metre (Station 3, September 1987) are shown in

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Fig. 6. Abundance of predominant macrofauna taxa in the uppermost sediment layer of the test area in September 1987 [Ind./sample, sample size = 18.1 cm2], the five x-axes represent five neighbouring rows of corers (1-5). 188

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Fig. 7. Abundance of predominant meiofauna taxa in the uppermost sediment layer (lst cm) sampled in July (station 2, 1st row, n = 4 samples A, B, C, E) and in September 1987 (station 3, 1 sample); y = [Ind./sample, sample size = 18.1 cm2].

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Fig. 8. Distribution of contents of chlorophyll-a (a) and CPE (b) in the 1st cm layer at station 3 in September 1987, n = 36 samples [gg cm-2]. Axes give sample coordinates.

Fig. 8. Actinocyclus normanii (GREGORY), a diatom characteristic of estuarine plankton, appeared as predominant algal species (ca. 95% of living cells) in the samples. Staining of samples indicated largely living algal populations at time of sampling. Mean values of chlorophyll-a and CPE content in all five sediment layers showed a maximum in the 2rid centimetre, with the 5th centimetre value lying below the 1st

centimetre value (Table 1). Mean fluctuations of values range from 19-47% for chlorophyll-a and 17-43% for CPE. Chlorophyll-a values of the 1st centimetre showed regular distribution, while statistics of CPE values indicate patchy distribution (g 2, p < 0.05). Statistics of chlorophyll-a and CPE values in all other layers revealed patchy distribution ( Z 2, p < 0.01).

Table 1. Statistics of chlorophyll-a and CPE-contents in five sediment layers on the sample area of station 3 (September 1987). Limit values for Z2 (n = 36) are: ~234;0.05 = 48.60; )~234;0.01 = 65.25. 5. cm (n = 26): Z22~:0.0~= 51.18. C/P gives the ratio of chlorophyll-a and phaeopigments.

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49.9 113.9 148.7 80.7 203.6

1.14 1.08 1.14 1.12 1.09

Table 2. Correlation analysis (Spearman ranking correlation test) of distribution patterns of various taxa and parameters recorded from the

first sediment centimetre in September samples; *Turbellaria were absent in July samples ( • = significant at p _<0.05).

Chlorophyll-a Oligochaeta Chironomidae Cladocera Turbellaria*

CPE

Chlorophyll-a

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Cladocera

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Mean values of each four neighbouring samples were compared to faunal counts using the Spearman ranking correlation test. Results of correlation analysis of distribution patterns of macrofauna taxa and chloroplastic pigment parameters recorded from the first sediment centimetre in September samples are surveyed in Table 2. Results indicate significant correlations (p < 0.05) between distribution patterns of chloroplastic pigments (algae, diatoms), Oligochaeta and Chironomidae.

5. Discussion Benthologists are familiar with patchiness in the benthic environment. Most populations show considerable inhomogenity in their spatial distribution. Based on this presumption, the present study intended to investigate interrelations of environmental conditions and the distribution patterns of the organisms community on a small-scale sample area. The set of parameters studied (character and homogeneity of sediment features, distribution and abundance of major faunal taxa, distribution of chloroplastic pigments as an indicator of benthic primary production and food resources available) is not exhaustive, but allows a first characterisation of the benthic environment of freshwater tidal mud flat. Sediment parameters showed no significant inhomogeneity of distribution within an area of the sample size (25 x 25 cm). Distribution of macrofauna taxa (Oligochatae, Chironomidae, Cladocera, Turbellaria) appeared to be controlled by oxygen availability (high abundance in uppermost sediment layer), sediment surface morphology (remaining rest water puddles at low tide) and possibly preypredator relationships. Distribution of chloroplastic pigments demonstrated short-term dynamics caused by allochthonous input from the plankton on the sediment surface and patchy distribution of endobenthic algal production, most likely reflecting differences and dynamics in nutrient availability (e.g. KERNER et al. 1986; CARION 8L BLANCHARD 1994). However, further detailed studies would be needed to quantify or model the dynamics of this multifactorial system. For reason of the limited data base from this study results allow only rough insight into the web of relations between the abiotic conditions and various groups of living organisms in the highly unique freshwater tidal flats. Nevertheless, in a number of cases present observations agree with results of other studies, which concentrated on selected aspects of the complex system of relevant factors in comparable environments (see below). Environmental conditions in freshwater tidal flats largely resemble those of marine tidal mud flats (e.g. grain size composition, temperature and oxygen conditions etc.). The benthic fauna community appears to be largely restricted to endobenthic freshwater species. Nevertheless, observed concentration of Cladocera in the 1st sediment centimetre and 190

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their clumsy motion within the sediment suggest at least partly epibenthic life strategy. Patterns of sediment parameters analysed from July samples showed regular distribution for water content, organic content, grain size medians and sorting coefficients (for detailed results compare REINICI~ 1988). However, depending on the water movement deposition and erosion of sediments significantly control the environment. Deposition rates of 2-9 c m y r -l had been reported by DORJES & REINECK(1981) from the MiJhlenberger Loch (River Elbe). The following process of colonisation, successive stabilisation and reworking of sediments by particle aggregation, bacterial and algal activities (mucus and mat-formation) and bioturbation (formation of faecal pellets) generates a unique environment colonised by an assemblage of specialised freshwater benthic macro- and meiofauna species (e.g. FIGGEet al. 1980; PENNAK 1985). PFANNKUCHEet al. (1975) surveyed the benthic fauna occurring in the nearby freshwater tidal flats of "F~ihrmannsand", PFANNKUCHE(1981) extensively describes the Oligochaeta fauna of the same location. The fauna is generally characterised by eurytopic freshwater species, especially saprobic species. Species are opportunistic colonisers of largely ephemeral habitats with high reproduction rates and often distinct annual cycles, spreading in large numbers as planktonic larvae or through drift. Solar warming of the dark-grey sediments (up to 30 °C in July 1987) as well as frost wind or ice in winter mark the possible temperature range. Oxygen supply into the sediment appeared to be limited to the uppermost few millimetres (0.8-1.8 cm depth), with black sulfidic sediments found below. Abundant endobenthic fauna appeared to be restricted to the uppermost centimetre of the sediment surface, with Oligochaeta, Chironomidae, cladoceran Phyllopoda and Turbellaria dominating the macrofauna, and Nematoda, Rotatoria, Gastrotricha and Tardigrada reaching large abundances in the meiofanna. In a study from tidal flats in the nearby "Pagensander Nebenelbe" SCHUMACHER (1963) demonstrated a strong correlation between abundance of Tubificidae (Oligochaeta) and the thickness of the oxidised layer of sediment. Relative abundances of the two predominant cladoceran species, Leydigia acanthocercoides and Iliocryptus sordidus, changed drastically between both sample dates. Total abundance increased 7-fold during summer (July to September). Assuming sufficient oxygen due to regular exposure to the air (diffusion into upper sediment layers) and abundant food supply to detritus feeding Cladocera at least during the population maximum in autumn (L. acanthocercoides, WHITESIDE et al. 1978), limited space might be the factor controlling distribution patterns of individuals. Patchy distribution of both species was highly significant in both sample sets. In the July samples distribution patterns of Cladocera significantly correlated with the Oligochaeta, abundance minima largely matched with the patterns of overlaying rest water puddles.

Abundances of Chironomidae and Turbellaria in September were significantly lower than those of Oligochaeta and Cladocera. Correlation of their respective distribution patterns with Oligochaeta and Cladocera indicates certain predator-prey relationships. From laboratory experiments and gut contents analysis of free-living species LODEN (1974) proved predation of Tanipodinae and Chironomidae on Oligochaeta. GOULDEN(1971) described the break-down of a cladoceran (Chydoridae) spring population from predation by 3rd and 4th instars of tanipodine Chironomidae in Canadian lake sediments. Mainly predatory Turbellaria might influence abundance of Cladocera at least during the population maximum in autumn. Further, epibenthic predation by fish during high tides or by birds during low tide periods might play eminent roles within this benthic network context. However, for quantitative evidence of these relations more experimental data are needed. The sediment surface was largely covered by unicellular algae (diatoms) forming extended layers during low water periods. The predominant species in the samples was Actinocyclus normanii (GREGORY).This species is described to be typical of the phytoplankton of the Elbe and Weser estuaries (after HUSTEDT 1957), reaching high abundance on sediment surfaces after sedimentation (HECKMAN1985). The high proportion of living diatom cells in September indicates strong sedimentation during preceding days - which is naturally supported by low current velocities within the bight. Regular distribution of chlorophyll-a values within the 1st sediment centimetre of the test area supports this interpretation, while chlorophyll-a and CPE values showed patchy patterns in all other sediment layers. This patchiness might be related to small-scale differences in nutrient contents of pore water. Especially sediment feeding Tubificidae are well known for their bioturbative activities, contributing to nutrient and oxygen turnover of deeper sediment layers, as do the larvae of Chironomidae (e.g. FUKUHARA~; SAKAMOTO1987, 1988; GRANfiLI1978). This interpretation receives additional support by the correlation between distribution patterns of Oligochaeta and Chironomidae with chlorophyll-a and CPE. A reason for the maximum of pigment content in the 2 ndsediment centimetre can only be speculated from present data. Lower limits of the oxygenated sediment horizon at 0.8-1.8 cm might result in a concentration of moving diatom cells in this respective horizon (compare HECKMAN1985). However, low-water conditions during sampling and upward moving of diatom cells would counteract this effect. Habitat conditions appear to be rather constant at least during summer months. However, benthos composition appears dynamic and changing over the year. Conditions allow colonisation by benthic fauna of highly opportunistic taxa only. Controlling factors on a large scale are the sediment dynamics and allochthonous nutrient supply, linking the freshwater tidal sediment beds to the open water compartment of the river. On the small scale, patterns of faunal distribution appear to be related to local surface morphology, oxygen

availability and prey-predator relationships, besides seasonal abundance dynamics possibly superimposing competition for space or food. The freshwater tidal mud-flat system appears as a unique and fugitive system with highly specific conditions. This specificity makes this type of environment an important facet in estuary ecosystems and an interesting study object. However, the dynamic relations between different compartments of the system yet need more detailed and experimental studies. The present study reports a methodological approach to integrate the quasi-synchronous recording of different parameters involved in the network of acting parameters. Results demonstrate agreement of general characteristics with other benthic systems, as well as highlight specific features of the freshwater tidal flat habitats and dynamics of their community.

Acknowledgements: This study was generously supervised and supported by Prof. Dr. H. KAUSCH,who provided working facilities and valuable advice. Field works and data analysis were carried out in co-operation with Dr. R. VORBERG.Thanks are due to the colleagues in the Institute of Hydrobiology and Fisheries Science of the University of Hamburg, in particular to G. MAASERand A. HAGGE for continuous discussions and endless patience in advice. An anonymous reviewer provided valuable comments on the manuscript. U. GERBERof the company Ffitsch GmbH invited the analysis of sediment samples using the "Laser-Particle-Sizer" in their laboratory in Idar-Oberstein. The Naturschutzamt of the UmweltbehOrde Hamburg permitted access to the protected island NeBsand, and Mr. HALADYN,the island warden, kindly ferried us over and supported our work. Thanks are extended to my parents and all friends and colleagues, who made this study successful with their support.

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Author's address: Dr. GOTZ B. REINICKE, German Oceanographic Museum - Deutsches Meeresmuseum, Katharinenberg 14/20, D 18439 Stralsund, Germany; e-mail: goetz.reinicke @meeresmuseum.de