Are Wadden Sea tidal systems with a higher tidal range more resilient against sea level rise?

Accelerated sea level rise may have serious implications for the Wadden Sea ecosystem in its present state. If sediment accumulation rates on the extensive intertidal flats stay behind sea level rise, the flats will eventually submerge. Drowning of the flats has negative consequences for nature conservation and for coastal risk management. Based upon an evaluation of steady state relations for Wadden Sea tidal basins, Hofstede (Zeitschrift für Geomorphologie 59(3): 377-391, 2015) postulated that the capacity of these basins to balance sea level rise by accumulation on intertidal flats seems positively related to mean tidal range. In the present study, morphodynamical simulations with a numerical model were performed for two tidal basins in the German Wadden Sea to verify the empirically established hypothesis. The following conclusions are established. Larger mean tidal range improves the capacity of Wadden Sea tidal basins to balance sea level rise. Wadden Sea intertidal flats are effective sediment sinks and seem quite resilient against (higher rates of) sea level rise. Finally, subtidal gullies may constitute a significant sediment source for accumulation on intertidal flats in response to sea level rise. With respect to the limited comparability of the two investigated tidal systems, morphodynamical modelling of all Wadden Sea tidal systems should be conducted.     

Rhythmic motor activity responses of the California fiddler crab Uca crenulata to artificial light conditions

Uca crenulata, the California species of fiddler crab, was exposed to artificial light conditions to separate the influence of the light cycle from that of the tidal input on its rhythmic motor activity. Under both constant light and light-dark cycles, rhythmic activity was demonstrated in only 50% of the experimental crabs; the activity of the remaining 50% was random. Individuals exposed to constant light conditions after having been subjected to 24 h light-dark cycles demonstrated no significant difference in period length of their rhythmic activity from crabs investigated in constant light immediately after field collection. The mean period did not differ significantly from the tidal period of 24.8 h, but the variation was considerable. In artificial light-dark cycles, the activity rhythms were usually masked but, in some cases, synchronized. The results indicate that U. crenulata has an endogenous rhythm with a period close to the tidal cycle which may be synchronized by light as well as by tidal cues. The display of this endogenous rhythm, however, is poor.     

Synergistic effects of ultraviolet radiation and conditions at low tide on egg masses of limpets (<Emphasis Type="Italic">Benhamina obliquata</Emphasis> and <Emphasis Type="Italic">Siphonaria australis</Emphasis>) in New Zealand

The effects of ultraviolet radiation (UVR), desiccation and conditions in tidal pools on embryonic survival were examined for two common pulmonate limpets that lay intertidal benthic egg masses on rocky shores in New Zealand: Benhamina obliquata and Siphonaria australis. Field surveys and manipulative experiments were conducted between December 2006 and September 2007 in the Wellington region of New Zealand (41°17′S, 174°47′E). Egg mass deposition sites in the field were species-specific: B. obliquata deposited eggs primarily in shaded crevices, whereas S. australis predominantly deposited egg masses in the sun and in tidal pools. For both species, however, embryonic mortality was greater in egg masses that had been in full sun compared to shade. For S. australis, there was also high mortality in egg masses in tidal pools or desiccated compared to those that remained submerged in flowing seawater at low tide. In outdoor experiments, embryonic mortality was also always greatest for egg masses exposed to full sun, and lowest for those in shaded treatments. Mortality was also higher if egg masses were in simulated tidal pools, and for S. australis, if desiccated, compared to those submerged in flowing seawater. Periods of particularly sunny conditions with high temperatures also resulted in higher overall mortality. Finally, egg masses of both species that were initially deposited in the shade had greater mortality in response to subsequent UV exposure compared to egg masses initially deposited in full sun. Results from this study suggest that the egg masses of these two species are highly vulnerable to UVR, as well as other intertidal stressors. Embryos of both of these species may be at risk of high mortality particularly during summer when extreme conditions of UV intensity and high temperature coincide with low tide cycles. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Tidal and circadian activity rhythms in juvenile plaice,Pleuronectes platessa

Young plaice (Pleuronectes platessa L.), caught in the intertidal zone, exhibit a short-lived tidal rhythm when their activity is recorded in darkness in the laboratory. This tidal rhythm rapidly changes to one of circadian frequency both in darkness and in light-dark cycles. It is considered that the basic rhythm is circadian in nature, but that it can be entrained to keep in phase with the tides by some, as yet unknown, Zeitgeber present under tidal conditions.     

The “piece-by-piece predation” of<Emphasis Type="Italic"> Eteone longa</Emphasis> on<Emphasis Type="Italic"> Scolelepis squamata</Emphasis> (Polychaetes )—traces on the sediment documenting chase,defence and mutilation

On many sea shores of the Niedersachsen coast, the polychaete Scolelepis squamata is the dominant animal species living in the sediment of exposed beaches.The population of the predatory species Eteone longa, with a main distribution in more sheltered intertidal and subtidal habitats, has a certain overlap with S. squamata. In these restricted areas, S. squamata suffers from a permanent pursuit by E. longa during low tide. Field and laboratory studies have revealed that this predator-prey relationship follows a distinct behavioural pattern and is an exploitation of surviving animals: the predator does not ingest the whole prey individual but only feeds on regenerable parts of the body. Both species draw characteristic tracks on the sediment surface that illustrate the phases of the chase, the attempts at defence and the mutilation of the prey species.Communicated by O. Kinne, Oldendorf/Luhe     

Experimental field study of the effects of crude oil,drill cuttings and natural biodeposits on microphyto- and macrozoobenthic communities in a Mediterranean area

Effects of an experimental pollution by neogenous and fossil organic matter on microphyto- and macrozoobenthic communities were studied in situ over a 1 yr period (July 1989 to May 1990) in a shallow microtidal bay (Gulf of Fos, south coast of France). Three experimental enclosures of 1 m² non-defaunated sediments were covered with 1 cm of polluted defaunated sediments. The sediment in one enclosure contained natural biodeposits with a high organic matter content (BD), that in a second enclosure contained Arabian light crude oil (BAL), and the sediment in the third enclosure contained diesel oil-based cuttings (CUT). Pollution by contaminants did not prevent microphytobenthos from colouizing sediments. Population changes over time were quite similar in all enclosures, except in CUT, where a four times higher chlorophyll a content appeared to be related to a decreased number of grazers and consequently lower grazing rates of animals. Toxicity to the fauna was immediate in the case of BAL and occurred within 3 mo at CUT. Opportunistic species settled in all contaminated sediments; this occurred quite rapidly in BD and BAL which recovered within 3 mo to levels comparable with control sediments. In CUT, natural populations had not recovered after 1 yr, whereas a quasi-monospecific population of Capitella capitata was still present. During the first 3 mo, the oxygen demand of the sediment was higher in oil-contaminated sediments than in controls. On the whole, the changes in fluxes and organism assemblages in our weakly tidal area appear to be consistent with other findings in macrotidal seas.     

Habitats and tidal accessibility of the marine foods of dabbling ducks and brant in Boundary Bay,British Columbia

The role of birds in intertidal food webs is mediated by effects of tides on the habitat, distribution, and accessibility of food organisms. We studied effects of elevation, sediment characteristics, vegetation, and tidal cycles on waterfowl feeding and food organisms in Boundary Bay, British Columbia, Canada from September to March 1990–1992. Percent dry mass of esophageal contents was mainly Zostera japonica leaves (84%) for American wigeon (Anas americana Gmelin), and leaves of Z. japonica (57%) and Z. marina (41%) for brant (Branta bernicla L.). Northern pintails (A. acuta L.) and mallards (A. platyrhynchos L.) ate mostly the leaves, seeds, and rhizomes of Z. japonica (48 and 72%, respectively) and gastropods (12 and 28%). Green-winged teal (A. crecca L.) ate mainly amphipods (68%) and saltmarsh seeds (19%). The well-sorted fine sand (median 0.125 mm) to medium sand (0.25 mm), and the low organic carbon content (1.31±0.12% dry mass), did not appear to vary sufficiently along 2.1- and 2.6-km transects to affect macroinvertebrate distributions. Three habitat zones were recognized: (1) upper intertidal (sandflats); (2) mid-intertidal (largely monotypic Z. japonica); and (3) lower intertidal (mixed Z. japonica and Z. marina). These zones, although related to elevation, had greater influence on amphipod distributions than did elevation alone or detrital biomass. Biomass and numbers of most invertebrate foods of waterfowl were greatest in the eelgrass zones, with the upper elevational limit of Z. japonica occurring at about Mean Water Level (MWL). Accordingly, wigeon, pintails, and mallards fed mostly below MWL. Because of seasonal tidal cycles, daily accessibility of areas below MWL but within reach of surfacefeeding waterfowl decreased from a mean of 10.8 h in October to 9.6 h in December. In October only 10 d and <12 h available feeding time, whereas in December and January every day had <12 h available feeding time. Seasonal effects of tides on accessibility might limit sufficient feeding by some waterfowl species in intertidal areas, increasing their reliance on alternative feeding sites.     

Habitat harshness and morphodynamics: life history traits of the mole crab Emerita brasiliensis in Uruguayan sandy beaches

The habitat harshness hypothesis (HHH) postulates that in reflective beaches the harsh environment forces organisms to divert more energy towards maintenance and they therefore have lower abundance, fecundity, growth and survival rates than in dissipative beaches. Recent investigations have tested this hypothesis through single comparisons of only two beaches, and thus the observed trends in population level variables cannot be attributed incontestably to the beach state, but only to location. Here, abundance, reproduction, recruitment, population structure and body size of the intertidal mole crab Emerita brasiliensis were compared between populations from eight microtidal exposed sandy beaches with contrasting morphodynamics, sampled bimonthly during 22 months throughout the 180 km Uruguayan Atlantic coast. Physical variables and compound indices of the beach state were used to categorize sandy beaches. The results of this bi-annual large-scale analysis were fully consistent with the predictions of the HHH: abundance (total and population components), duration of the reproduction and recruitment seasons and the individual size of megalops and females of the mole crab E. brasiliensis decreased from dissipative to reflective beaches. This was reflected by linear or, mostly, nonlinear relationships between biological and both physical variables and compound indices of beach state. In conclusion, this multi-beach sampling provides compelling evidence of a consistent response of demographic and life history traits of an intertidal beach species to morphodynamic characteristics.     

Predictors of specialist avifaunal decline in coastal marshes

Coastal marshes are one of the world's most productive ecosystems. Consequently, they have been heavily used by humans for centuries, resulting in ecosystem loss. Direct human modifications such as road crossings and ditches and climatic stressors such as sea‐level rise and extreme storm events have the potential to further degrade the quantity and quality of marsh along coastlines. We used an 18‐year marsh‐bird database to generate population trends for 5 avian species (Rallus crepitans, Tringa semipalmata semipalmata, Ammodramus nelsonii subvirgatus, Ammodramus caudacutus, and Ammodramus maritimus) that breed almost exclusively in tidal marshes, and are potentially vulnerable to marsh degradation and loss as a result of anthropogenic change. We generated community and species trends across 3 spatial scales and explored possible drivers of the changes we observed, including marsh ditching, tidal restriction through road crossings, local rates of sea‐level rise, and potential for extreme flooding events. The specialist community showed negative trends in tidally restricted marshes (?2.4% annually from 1998 to 2012) but was stable in unrestricted marshes across the same period. At the species level, we found negative population trends in 3 of the 5 specialist species, ranging from ?4.2% to 9.0% annually. We suggest that tidal restriction may accelerate degradation of tidal marsh resilience to sea‐level rise by limiting sediment supply necessary for marsh accretion, resulting in specialist habitat loss in tidally restricted marshes. Based on our findings, we predict a collapse of the global population of Saltmarsh Sparrows (A. caudacutus) within the next 50 years and suggest that immediate conservation action is needed to prevent extinction of this species. We also suggest mitigation actions to restore sediment supply to coastal marshes to help sustain this ecosystem into the future.     

Geographical patterns in reproductive biology of the Pan-American sandy beach isopod<Emphasis Type="Italic"> Excirolana braziliensis</Emphasis>

Eleven populations of the Pan-American sandy beach isopod Excirolana braziliensis, distributed from tropical (9°N) to temperate (39°S) sandy beaches in Atlantic and Pacific Oceans, were analyzed to determine latitudinal variations in: breeding and recruitment patterns, sex ratios, size of ovigerous females and of juveniles and length–fecundity relationships. E. braziliensis exhibited strong latitudinal patterns in all reproductive traits throughout its distribution range. Breeding and recruitment shifted from continuous to seasonal from tropical to temperate beaches, having a predominance of females at higher latitudes. In agreement with the latitudinal gradient hypothesis, ovigerous females inhabiting tropical sandy beaches (low latitudes) were smaller, became sexually mature at smaller sizes and had lower individual fecundity than on temperate beaches. Juveniles were also smallest at low latitudes. Between-ocean comparisons showed very similar reproductive characteristics for roughly the same latitude. These linked reproductive parameters suggest that the intensity of breeding effort is associated with the duration of the breeding season and geographically size-related characteristics of the species. Geographic variations in the breeding and recruitment seasons, as well as in individual fecundity, size structure of mature females and sex ratios, are proposed to have major consequences in explaining local variations in population demography. Our paper also reinforces the notion that sandy beach animals are highly plastic.Communicated by O. Kinne, Oldendorf/Luhe     

Rhythmicity of larval release in three species of intertidal brachyuran crabs (Crustacea: Brachyura) from Inhaca Island (Mozambique)

 A study of rhythmicity of larval release in three species of intertidal brachyuran crabs, based on laboratory and field experiments, was undertaken at Inhaca Island, southern Mozambique, using Leptodius exaratus and Macrophthalmus grandidieri from December 1994 to January 1995, and Arcotheres palaensis from April to July 1995. L. exaratus and M. grandidieri showed a semi-lunar cycle in larval release. The release of larvae for L. exaratus, a species having conspicuous larvae, occurred in the first half of the night, after the post-crepuscular high tide, which suggests maximisation of protection of larvae from visual predation. The larval release activity matched the late spring and early neap tides. Results from the field were similar to those from the laboratory. M. grandidieri, having inconspicuous larvae, did not show a pattern related to the light–dark cycle and hatched during spring tides (around full and new moons) to maximise larval dispersion. A. palaensis, living inside the host mussel which inhabits the lowest section of the intertidal zone, did not show a relation with moon phase, tidal or light–dark cycles. Received: 16 February 1999 / Accepted: 8 December 1999     

Chemical defense in the eggs and embryos of antarctic sea stars (Echinodermata)

Fifteen species of reproductively mature echinoderms (11 sea stars, 3 sea urchins, 1 sea cucumber) were collected from McMurdo Sound, Antarctica, during austral spring and summer of 1985 and 1986; eggs and embryos were obtained, and were tested for ichthyonoxicity using the common marine killifishFundulus grandis as a model predator. Chemical deterrents occurred in the large, yolky eggs of the pelagic lecithotrophic sea starPerknaster fuscus and the planktotrophic sea starPorania antarctica. Brooded embryos of the sea starsDiplasterias brucei andNotasterias armata were also noxious. Significant ichthyonoxicity was not detected in the remaining 7 species of sea stars, 3 sea urchins, and 1 sea cucumber. Chemical deterrents were generally effective at concentrations below a single egg or embryo per agar test-pellet. Although chemicals found in these eggs and embryos are noxious to an allopatric fish, they may not be noxious to sympatric fish.     

Measurement of chlorophyll fluorescence reveals mechanisms for habitat niche separation of the intertidal seagrasses <Emphasis Type="Italic">Thalassia hemprichii</Emphasis> and <Emphasis Type="Italic">Halodule uninervis</Emphasis>

In Taiwan, Thalassia hemprichii dominates the upper intertidal zone, whereas Halodule uninervis occupies the lower intertidal zone. We tested the hypothesis that T. hemprichii is better adapted to high irradiance and more resistant to air exposure than H. uninervis. The photosynthetic efficiency, damage, and extent of recovery were determined by measuring chlorophyll fluorescence using pulse amplitude modulated fluorometry. Both species growing in tidal pools, in response to high irradiance alone, revealed a small depression in maximal quantum yield of photosystem II (Fv/Fm) at noon. The second experiment compared the effect of air exposure alone and the combined effect of air exposure with high irradiance by interposing a shading screen on both species, growing in the intertidal zone over a diurnal cycle. Values of Fv/Fm of both the shaded and irradiated T. hemprichii remained high at low tide. However, H. uninervis exhibited a marked depression following air exposure and a synergistic depression under the combined effect. The experimental manipulations of exposure time demonstrated that the tolerance of T. hemprichii to the combined effect was longer and the recovery from air exposure following re-submersion was better than those of H. uninervis. Both species were more susceptible to the combined effect in the dry season than in the wet season. Our results suggest that air exposure is more important than high irradiance in constraining the distribution of H. uninervis in the upper intertidal zone. This was confirmed by transplantation experiments in which a rapid decline of H. uninervis was observed after transplantation into the upper intertidal zone. In the lower intertidal zone, measurements of the response of the photosynthetic electron transport rate to irradiance demonstrated that the transplanted T. hemprichii exhibited a sun-type response and H. uninervis a shade-type response.     

Morphological divergence of Eastern Pacific and Caribbean isopods: effects of a land barrier and the Panama Canal

The Isthmus of Panama rose approximately 3 million years before the present (mybp) and isolated biotas in the tropical eastern Pacific from those in the Caribbean Sea. Populations that were split by the Isthmus and have evolved in allopatry since that time are known as geminates. The surf zone/beach isopod Excirolana braziliensis Richardson was examined between 1984 and 1989 to test the hypothesis that divergence in geminate isopod morphology has occurred, and that geminate divergence is greater than divergence between local populations from the same coastline. Three morphs of Excirolana braziliensis, one in the Caribbean and two in the eastern Pacific, were discovered using numerical taxonomic methods that adjust for body size. The two Pacific morphs have overlapping large-scale distributions, but those morphs are segregated on a smaller scale by beach. We inferred that one Pacific morph and one Caribbean morph were geminates, based on their relative similarity in shape, their geographical ranges, and natural history information about the organism's dispersal capabilities. The origin of the third morph probably predates the Isthmus of Panama, given its relative dissimilarity from the geminate morphs. The presumed geminates differ primarily with respect to the rostrum, antennae and one male reproductive structure. Divergence between geminates is greater than divergence between local populations of any morph along a coastline. Because only one morph occurs in the Caribbean, that region contains less morphological variation than the eastern Pacific, which contains two morphs. There was weak evidence that some introductions may have taken place in the last century from the Caribbean to the Pacific; however, introductions have not masked the pattern of divergence that has developed over millions of years.     

Tidal variations in the photosynthesis of estuarine phytoplankton isolated in a tank

In 1981 two large (1 200 1) seawater samples from the St. Lawrence Estuary were kept under constant temperature and light conditions for periods of 50 and 68 h, respectively. In both tank experiments, semidiurnal variations in NH₄ were observed that could be related to cyclical NH₄ uptake by the phytoplankton. Semidiurnal cycles in photosynthetic efficiency (^B) and intracellular chlorophyll a in the tank, phased on tides at sea, were also evidenced in both experiments. These results support the hypothesis that variations in phytoplankton photosynthetic activity, which are possibly endogenous, can be phased on semidiurnal variations in vertical tidal mixing (variations of the mean light conditions in the mixed layer). In addition, observed variations in intracellular chlorophyll a suggest the possibility of endogenous cycles of phytoplankton light and shade adaptation.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Factors limiting distribution and activity patterns of the soldier crab Dotilla myctiroides in Phuket, South Thailand

This study was carried out between January and March 1995 on the intertidal sand flats of Tang Khen Bay, Phuket, South Thailand, where the soldier crab Dotilla myctiroides (H. Milne-Edwards) occurs in densities of up to 120 m⁻². In this bay, long, ribbon-like sand waves (wavelength 40 m, height 0.4 m) are interspersed with shallow pools, running approximately parallel to the shore. During daylight low-tides, exposure of the sand waves is followed 15 to 20 min later by the emergence of the crabs which have been buried under the sediment surface during high tide. Their subsequent burrowing and feeding activity results in the production of large numbers of sand pellets on the sediment surface. Most crabs retreat down their burrows, and some also plug the burrow entrance, prior to being covered by the incoming tide. The crab burrows have a distinct distribution on the sand waves. Burrows are most dense at the top of each sand wave, and a band of unburrowed sediment adjoins the adjacent tidal pools. Crabs are most abundant between mean high-water neap-tide level and mean low-water neap-tide level, where the median particle size of the surface sediment is ∼2 . Measurements of water-table depth below the sand waves and the exposure time of the sediment indicated that, where sediment size is suitable, the main factor controlling crab distribution is the duration of daytime exposure. This observation is in contrast to those of many previous studies, which have suggested that water-table height and sediment drainage are the main factors controlling the distribution of D. myctiroides. Received: 14 January 1998 / Accepted: 6 May 1999     

Nitrogen fixation associated with the cyanobacterial mat of a marine laminated microbial ecosystem

The nitrogenase activity in the cyanobacterial mat of a laminated microbial ecosystem was investigated by the acetylene reduction method. Measurements under several conditions such as light and dark, aerobic and anaerobic and by inhibiting photosystem II by 10^-5 M DCMU showed the nitrogenase activity to be light stimulated and to some degree inhibited by oxygen. An appreciable amount of activity was also present under complete aerobic conditions. We estimated 8 to 15 kg N fixed per hectare per year for that part of the intertidal flat supporting growth of cyanobacteria. By measuring a vertical sediment profile, nitrogenase was shown to be associated with the cyanobacterial mat. Diurnal measurements of nitrogenase showed two activity peaks, one at sunrise and one at sunset. Following population dynamics in the cyanobacterial mat showed Microcoleus sp., Oscillatoria spp., Spirulina sp., Gloeocapsa sp. and sometimes Merismopedia sp. to be present. During four years of observations we never found any heterocystous cyanobacteria. Non-heterocystous cyanobacteria apparently play an important role in nitrogen fixation in this marine intertidal environment.     

Macrozoobenthic community structure in the Ría de Foz,an intertidal estuary (Galicia,Northwest Spain)

The community structure of the macrobenthic (>1 mm) fauna of an intertidal soft-bottom estuary, the Ría de Foz (Galicia, Northwest Spain) was investigated on 99 occasions during June, September and December 1984 and March 1985. The infaunal data were subjected to classification and ordination techniques. The analyses indicated that sediment characteristics (grain size, percentage silt-clay and organic-matter content) and tidal height are the most important factors governing the distribution and abundance of the intertidal communities. Three faunal assemblages were identified and examined in terms of species dominance, constancy and fidelity. In the clean sands of the outer estuary two main assemblages were distinguished; one, corresponding to the boreal LusitanianTellina community, inhabited the middle and lower tidal levels; the other, typical of a biocoenosis of sands and slightly muddy mediolittoral sands, inhabited the upper tidal level. The third benthic assemblage occupied the muddy sands and mud of the inner estuary and displayed some features in common with theCardium edule-Scrobicularia community described by Thorson.     

Niche segregation and habitat specialisation of harpacticoid copepods in a tropical seagrass bed

Several harpacticoid copepod species are adapted to an epiphytic lifestyle. Previous studies on tropical seagrass meiofauna mainly focussed on the epiphytic communities and neglected the benthic component. The present study aims to document the benthic harpacticoid copepod communities sampled from different sediment depth horizons adjacent to five seagrass species in the intertidal and subtidal zone of a tropical seagrass bed (Gazi Bay, Kenya). Two benthic copepod communities could be identified mainly based on the tidal position of the samples: a first community was collected near the intertidal seagrasses Halophila ovalis and Halodule wrightii; a second community occurred near the subtidal seagrasses Thalassia hemprichii, Syringodium isoetifolium and Halophila stipulacea. The first community was mainly determined by sediment characteristics (e.g. skewness), while the second community was split off based on organic matter content (% TOM), nutrient and pigment values. A subtle combination of horizontal and vertical niche segregation was reported for the dominant copepod families. Species of the families Thalestridae, Laophontidae and Diosaccidae were structured by tidal position and showed a strong preference for the subtidal zone. The opposite strategy, i.e. a clear preference for the intertidal zone, was found for copepods belonging to the families Paramesochridae and Canuellidae. In addition, Apodopsyllus africanus (Paramesochridae) was well-adapted to stress and was concentrated in the deeper sediment layers near the subtidal seagrasses. On the other hand, Canuellidae, as filter feeders, were concentrated in the upper centimetres of the sediment. The families Ectinosomatidae and Cletodidae did not show any vertical or horizontal segregation. On the species level, however, clear horizontal niche segregation was detected for the family Cletodidae. In addition to the reported ecological results, the study material was used to evaluate different niche definitions. We found tidal position to be the most important factor forcing harpacticoids to specialise. Sediment depth horizon was less powerful in dividing the families into different guilds (from specialists to generalists) based on standardised niche breadth. The present study documents the subtle habitat partitioning of co-existing species in a limited area and its role in sustaining high biodiversity in the community.