Crustacean Densities Research Articles

The Impact of the Macrophyte Substratum and Season on Crustacean Zooplankton Communities of Three Shallow and Macrophyte-Dominated Lakes

ABSTRACT I conducted a three-season study on three lakes in three different habitats (Chara tomentosa bed, Typha angustifolia stand, and the open water) in order to determine the best predictors of microcrustacean distribution. The seasonal composition and abundance of crustacean zooplankters were typical of the pelagic zone in temperate lakes, and there was no appreciable difference in the zooplankton communities among the lakes. The lowest species richness and total abundance were in the open water zone, while the highest richness and abundance were within the C. tomentosa stand. Moreover, two cladoceran species (Acroperus harpae and Chydorus sphaericus) were uniquely associated with C. tomentosa. Canonical correspondence analysis identified density of plant stems and oxygen and total organic carbon concentrations in the water as the most important factors structuring the distribution of the microcrustacean community. Stepwise multiple forward regression confirmed that plant stem length and biomass were biometric parameters that influenced crustacean density. Total nitrogen concentration and pH were implicated in the density of Ceriodaphnia pulchella, while temperature and conductivity influenced A. harpae abundance.

Long-term zooplankton body size and species changes in a subtropical lake: implications for lake management

Analysis of long-term (1970-2002) zooplankton body sizes in Lake Kinneret, as recorded in an histori- cal zooplankton sample archive, combined with re-analysis of published long-term zooplankton taxonomic abun- dance data revealed previously undetected patterns in zooplankton biomass and demonstrates a profound long-term impact of planktivorous fi sh predation intensity driven by fiharvest practices. During the 1970s, crustacean zooplankton biomass remained relatively constant at a mean (±SD) of 1.50 ± 0.26 g m -3 , dropped precipitously in the 1980s reaching 0.57 g m -3 by 1993, and rebounded to 1.33 ± 0.19 g m -3 in the early 2000s. The 1980s decline refl ects decreased cladoceran biomass due to a decline in mean body mass in the three dominant cladoceran genera (Diaphanosoma, Ceriodaphnia, Bosmina) and decreased copepod biomass that resulted from a shift in dominance by large Mesocyclops ogunnus to small Thermocyclops dybowskii. Although crustacean body sizes remained small through the early 2000s, cladoceran and copepod densities increased by ~34 and > 300 %, respectively, after 1994 leading to a dramatic increase in crustacean zooplankton biomass. These patterns indicate an intensifi cation in fi predation throughout the 1980s and 1990s, and although zooplankton biomass rebounded in 1996-2002 to ~80 % of the 1970s levels due to sharp increases in crustacean densities, smaller cladoceran and copepod body sizes sug- gest that Lake Kinneret zooplankton continue to be strongly infl uenced by intense fi sh predation.

Helminth communities of the ruddScardinius erythrophthalmus(Cypriniformes, Cyprinidae) from Srebarna Biosphere Reserve, Bulgaria

A total of 180 individuals of Scardinius erythrophthalmus from Srebarna Lake, north-east Bulgaria, were examined for helminth parasites during spring, summer and autumn of 2005-2006 (30 specimens per season). Helminth parasites were recorded in 166 individuals (92.2%). Ten helminth species were found: trematodes--metacercariae of Posthodiplostomum cuticola, Diplostomum chromatophorum and D. spathaceum; monogeneans--Dactylogyrus difformis, D. difformoides and Diplozoon scardinii; cestodes--metacestodes of Paradilepis scolecina; nematodes--larvae of Spiroxys contortus and Rhabdochona denudata and adults of Schulmanela petruschewskii. Significant seasonal differences in the mean abundance were recorded for D. difformis, D. difformoides and D. scardinii. Significant seasonal differences in prevalence were recorded for P. cuticola, D. difformis, D. difformoides and D. scardinii. Dominant species in the component communities were P. cuticola, D. difformis and D. difformoides. The species composition of the component community essentially differs from that in the adjacent Danube River; a possible reason is the specific hydrobiological situation of the lake, characterized by a low density of benthic molluscs and crustaceans.

Prey versus substrate as determinants of habitat choice in a feeding shorebird

Many shorebirds on their non-breeding grounds feed on macrobenthic fauna which become available at low tide in coastal intertidal flats. The Eastern Curlew Numenius madagascariensis in Moreton Bay Australia, varies greatly in density among different tidal flats. This study asks: how important is the abundance of intertidal prey as a predictor of this variation? We quantified feeding curlews’ diet across 12 sites (different tidal flats, each re-visited at least eight times), through 970 focal observations. We also estimated the abundance of total macrobenthic fauna, potential prey taxa and crustacean prey on each tidal flat; measured as the number of individuals and a relative biomass index per unit substrate surface area obtained from substrate core samples. We estimated curlew density at each site using low-tide surveys from every site visit. Curlew density showed a strong positive association with both the density and biomass of fauna and of potential prey ( r values all around 0.70) across the 12 flats. Associations with crustacean density and biomass were also statistically significant (r values both 0.60). However, these variables also showed a strong negative correlation with a measure of substrate resistance (based on the amount of hard material in the substrate core), which was the best predictor of curlew density ( r = −0.82). Curlews were most abundant at sites with the least resistant substrate, and these sites also generally had the highest faunal density and biomass. When the effect of substrate resistance was statistically removed, curlew density was no longer significantly correlated with fauna density and biomass. This suggests that macro-scale habitat choice by Eastern Curlew on their non-breeding grounds is more strongly influenced by prey availability (which is higher when substrate resistance is lower) than by prey density or biomass, although in Moreton Bay a positive correlation across sites meant that these factors were synergistic.

Spatiotemporal heterogeneity of plankton communities in Lake Donghu, China, as revealed by PCR–denaturing gradient gel electrophoresis and its relation to biotic and abiotic factors

The 16S and 18S rRNA genes of planktonic organisms derived from five stations with nutrient gradients in Lake Donghu, China, were studied by PCR-denaturing gradient gel electrophoresis (DGGE) fingerprinting, and the relationships between the genetic diversity of the plankton community and biotic/abiotic factors are discussed. The concentrations of total nitrogen (TN), total phosphorus (TP), NH(4)-N and As were found to be significantly related (P<0.05) to morphological composition of the plankton community. Both chemical and morphological analyses suggested that temporal heterogeneity was comparatively higher than spatial heterogeneity in Lake Donghu. Although the morphological composition was not identical to the DGGE fingerprints in characterizing habitat similarity, the two strongest eutrophic stations (I and II) were always initially grouped into one cluster. Canonical correspondence analysis suggested that the factors strongly correlated with the first two ordination axes were seasonally different. The concentrations of TN and TP and the densities of rotifers and crustaceans were generally the main factors related to the DGGE patterns of the plankton communities. The study suggested that genetic diversity as depicted by metagenomic techniques (such as PCR-DGGE fingerprinting) is a promising tool for ecological study of plankton communities and that such techniques are likely to play an increasingly important role in assessing the environmental conditions of aquatic habitats.

Foraging strategies involved in habitat use of shorebirds at the intertidal area of Chongming Dongtan, China

AbstractDense flocks of migratory shorebirds from diverse species often concentrate in the intertidal areas for stopover. Trophic structure, food partition, prey availability and selectivity, predation risk, and abiotic factors are often used to explain the differences in habitat use of coexisting shorebirds. We sampled the macrobenthos and surveyed the distribution of shorebird populations to study the effects of foraging strategies on the habitat use of shorebirds at Chongming Dongtan, an important stopover site for shorebirds on the East Asian–Australasian Flyway. Results show that the relative abundance of epifaunal macrobenthos in salt marshes was much higher than that in the bare flats, whereas the relative abundance of infaunal macrobenthos in salt marshes was much lower than that in bare flats. The relative abundance of two life forms of macrobenthos was similar in the transitional zones between the salt marshes and the bare flats. Shorebirds with different foraging strategies exhibited different habitat uses. Pause‐travel shorebirds mainly utilized the salt‐marsh fringes, while tactile continuous shorebirds relied heavily on the bare flats. There was no significant difference in habitat use for visual continuous shorebirds. The density of tactile continuous shorebirds was positively correlated with bivalve density, and that of visual continuous shorebirds positively with crustacean density. Meanwhile, the relative abundance of pause‐travel foraging shorebirds was positively correlated with the relative abundance of epifaunal, but negatively with infaunal macrobenthos. In contrast, the relative abundance of tactile foraging shorebirds had a positive correlation with infaunal but a negative one with epifaunal life form. Therefore, foraging strategies may play important roles in shorebirds’ habitat use in intertidal areas.

Habitat characteristics and spatial arrangement affecting the diversity of fish and decapod assemblages of seagrass ( Zostera marina) beds around the coast of Jersey (English Channel)

Recent research and management plans for seagrass habitats have called for landscape level approaches. The present study examines the spatial utilisation of subtidal seagrass beds by fish and decapods around the coast of Jersey (49°N 02° W). A hierarchical scale of landscape configuration and the plant characteristics of eight seagrass beds were measured and the contributions of these variables as predictors of the properties of the fish and decapod assemblages were evaluated using multiple linear regression models. The results indicated that total diversity had a negative relationship with transect heterogeneity and total species number had a weak negative association with increasing fragmentation. Both total diversity and total species number showed a positive relationship with depth. In fact, in all models of species number and densities, values were higher in deeper seagrass beds. Total decapod density increased with aggregation of seagrass patches within a landscape. In addition to landscape configuration, smaller-scale structural changes in both canopy height and epiphyte load appeared to influence densities of decapod crustaceans. At night, fewer patterns could be explained by the independent variables in the model.

Zooplankton of turbid and hydrologically dynamic prairie rivers

Summary1. Compared with rivers in more humid, forested ecoregions of eastern and midwestern U.S.A., rivers in semi‐arid grassland of the U.S. Great Plains tend to be relatively shallow, more variable in discharge, and characterised by high suspended sediment loads. Although critical life stages of fish in prairie rivers probably depend at least partially on zooplanktonic food, data on community and distributional patterns of potamoplankton in these widespread ecosystems are almost entirely absent.2. We examined summer zooplankton distribution in five prairie rivers (Arkansas, Kansas, Platte, Elkhorn, and Niobrara Rivers) spread over six degrees of latitude during 2003–2004. We compared our results from 126 samples with previously collected data from the Ohio and St Lawrence Rivers in forested ecoregions and correlated differences with abiotic environmental conditions.3. The importance of hydrological retention zones to stream biota has been recently demonstrated for rivers with quasi‐permanent islands and slackwater regions, but the importance of slackwaters formed by ephemeral sandbar islands in prairie rivers is unknown. We evaluated the role of hydrological retention for planktonic rotifers, cladocera, and copepods in the Kansas River during the summer of 2004.4. Zooplankton assemblages were extremely similar among prairie rivers (Sorensen Dissimilarity Index: mean = 0.07) but moderately disparate for comparisons of prairie versus forested‐basin rivers (mean = 0.50).5. Total zooplankton densities in prairie rivers (approximately 81 L−1) were intermediate between the Ohio (approximately 92 L−1) and St Lawrence Rivers (approximately 43 L−1), but relative abundances were significantly different. Rotifers represented &gt;99% of zooplankton individuals in grassland rivers, but only approximately 37–68% in other rivers. Rotifer species richness was lower in prairie rivers, but relative abundances of common genera were much less skewed compared with eastern rivers where Polyarthra dominated rotifer assemblages (41–73%).6. For comparisons among rivers, rotifers were significantly more abundant in turbid rivers, while microcrustaceans were less dense. However, for comparisons within the Kansas River over time, rotifer densities were inversely related to turbidity. We hypothesise that rotifers indirectly benefit from river turbidity because their food competitors (cladocera) and predators (e.g. cyclopoid copepods and visually feeding fish) are relatively more susceptible to suspended sediments.7. Crustacean densities were positively related to the degree of hydrological retention (negatively to current velocities) throughout the study, but rotifer densities were significantly depressed by current velocities only when river discharge was high, making slackwaters that much more valuable. Ephemeral sandbars may not provide sufficient hydrological retention in time and space to sustain viable crustacean populations, but they are adequate to help sustain growth of rotifer populations.

Effects of Rice Bran, Cottonseed Meal, and Alfalfa Meal on pH and Zooplankton

AbstractRice bran (RB), cottonseed meal (CSM), and alfalfa meal (ALF) were compared as organic fertilizers for their effects on pH and zooplankton abundance in limnocorrals that lacked fish. The weight (W) and nitrogen (N) content of CSM, which served as the control, were used to determine the other applications (RB‐N, RB‐W, ALF‐N and ALF‐W). Temperature, dissolved oxygen, chlorophyll a, ammonia nitrogen, and zooplankton densities were monitored for 8 weeks to simulate the average period for phase 1 production of striped bass Morone saxatilis (38–50 mm). Rice bran promoted better water quality than ALF or CSM when application rates were based on nitrogen content. The pH was lowest for RB‐N, intermediate for ALF‐N, and highest for CSM. Chlorophyll a was a function of nitrogen input, and the high‐nitrogen applications (CSM, ALF‐N, and RB‐N) supported similar phytoplankton standing crops that were 38–62% greater than the corresponding low‐nitrogen applications (ALF‐W and RB‐W). Water temperature and dissolved oxygen concentrations were similar among applications, and total ammonia nitrogen was below detectable levels for all applications. Overall mean densities of rotifers, nauplii, adult copepods, adult crustaceans, and total zooplankton did not differ significantly among applications. However, the mean density of cladocerans was significantly higher for RB‐N and ALF‐N than for ALF‐W. The densities of cladocerans, adult copepods, adult crustaceans, and total zooplankton correlated strongly with chlorophyll a but not with the quantities of fertilizers applied, indicating that phytoplankton probably was the primary food for these zooplankters. Based on pH, RB was better than CSM or ALF, but all had similar effects on zooplankton when application rates were based on nitrogen.

Niche overlap between juvenile flatfishes, Platichthys flesus and Solea solea, in a southern European estuary and adjacent coastal waters

Summary The distribution and abundance patterns as well as the feeding ecology of two flatfishes, Platichthys flesus (Linnaeus, 1758) and Solea solea (Linnaeus, 1758), were studied in the Douro estuary and adjacent coastal waters (Portugal) in order to evaluate the niche overlap between these species. Fish were collected during sampling surveys conducted every 2 months between November 2000 and March 2002 using a 12-m otter-trawl with 18 mm stretched mesh size, at 12 sites within the estuary and six sites in the adjacent coastal area. The highest densities of P. flesus were registered in the middle estuary; a peak density of 9.12 ind 1000 m−2 was obtained in October 2001. Solea solea densities were low compared with P. flesus; the highest values were also recorded in the middle estuary in October 2001. For both species, the proportion of juveniles was above 90% within the estuary. Highest densities of P. flesus and S. solea were recorded at sites with low water depth and high densities of crustaceans and polychaetes, the main food items in the diet of these species. High values of spatial and diet niche overlap were found between these two flatfishes (0.97 and 0.92, respectively).

Day–night variations in the demersal nekton assemblage on the Mediterranean shelf-break

The composition of the demersal nekton assemblage inhabiting the Mediterranean shelf-break of west-central Italy was examined to investigate the diel variation in species composition and abundance. Fish were obtained during four 24-h sampling periods that represented the four annual seasons. Thirty-two hauls (eight in each season) were conducted in a shelf-break area at 140–160 m depth. The density of fish, crustaceans, and decapods exhibited differences on short- and long-term time scales: multidimensional scaling suggested that season and light intensity may be important factors influencing fish assemblage composition. At the diel scale two main assemblages separating nocturnal and diurnal hauls were identified. The typical assemblage structure of shelf-break bottoms was represented during daylight by species such as Merluccius merluccius, Capros aper, Serranus hepatus, Macroramphosus scolopax and Trisopterus minutus capelanus. At night, the assemblage showed a drastic change due to the increase of small nektobenthic species, Argentina sphyraena and Glossanodon leioglossus, together with cave-dwelling fish ( Gnathophis mystax, Chlopsis bicolor, Nettastoma melanurum) and nektobenthic crustaceans (i.e., Parapenaeus longirostris and Solenocera membranacea). The above-mentioned differences in species composition may be related to changes in vertical distribution for trophic needs ( M. merluccius) and/or in the shoal structure ( A. sphyraena and G. leioglossus). The lack of data on the pattern of trawl catchability of Mediterranean species during the day may lead to an inaccurate estimation of species abundance when only diurnal trawl survey data are used.

An Integration of Water Physicochemistry, Algal Bioassays, Phytoplankton, and Zooplankton for Ecotoxicological Assessment in a Highly Polluted Lowland River

Results of different approaches potentially useful for the evaluation of water ambient quality were analysed and compared in a small temperate lowland river with mixed diffuse and multiple-point source pollution. The Reconquista River (Buenos Aires Province, Argentina), one of the most polluted watercourses of Latin America, receives agrochemicals as well as domestic and industrial (mostly untreated) effluents. Physical and chemical water variables were determined; unispecies algal bioassays (with Chlorella pyrenoidosa and Scenedesmus acutus) were carried out in laboratory; and density and structure of phyto- and zooplankton were analysed at three sites in four dates (representing a range of likely conditions in the river). A general scheme of association among plankton, bioassays and physical/chemical variables was elaborated, that helped to infer possible control factors in this multi-stressed system. Some empirical methods, but mainly mathematical ones including multivariate techniques (as PCA, cluster analysis), were applied for evaluation of samples. A preliminary selection of indices and attributes as potential indicators of the water river quality was made, and then applied for assay a tentative integrative ordination of samples. The relative best water quality was recorded when-where dissolved oxygen concentration, algal diversity and planktonic crustacean density were higher. The worst water quality corresponded to the lack of cladocerans and lowest crustacean density, and higher: organic and industrial pollution, major nutrients (ammonium and orthophosphates), BOD, hardness, conductivity, algal biomass in bioassays, phytoplankton density (>10 000 ind. mL-1), dominance of a single algal species (>90%), and rotifer proportion in zooplankton (>85%).

Recovery of floral and faunal communities after placement of dredged material on seagrasses in Laguna Madre, Texas

The objectives of this project were to determine how long alterations in habitat characteristics and use by fishery and forage organisms were detectable at dredged material placement sites in Laguna Madre, Texas. Water, sediment, seagrass, benthos, and nekton characteristics were measured and compared among newly deposited sediments and nearby and distant seagrasses each fall and spring over three years. Over this period, 75% of the estimated total surface area of the original deposits was either re-vegetated by seagrass or dispersed by winds and currents. Differences in water and sediment characteristics among habitat types were mostly detected early in the study. There were signs of steady seagrass re-colonization in the latter half of the study period, and mean seagrass coverage of deposits had reached 48% approximately three years after dredging. Clovergrass Halophila engelmannii was the initial colonist, but shoalgrass Halodule wrightii predominated after about one year. Densities of annelids and non-decapod crustaceans were generally significantly greater in close and distant seagrass habitats than in dredged material habitat, whereas densities of molluscs were not significantly related to habitat type. Nekton (fish and decapod) densities were almost always significantly greater in the two seagrass habitats than in dredged material deposits. Benthos and nekton communities in dredged material deposits were distinct from those in seagrass habitats. Recovery from dredged material placement was nearly complete for water column and sediment components after 1.5 to 3 years, but recovery of seagrasses, benthos, and nekton was predicted to take 4 to 8 years. The current 2 to 5 years dredging cycle virtually insures no time for ecosystem recovery before being disturbed again. The only way to ensure permanent protection of the high primary and secondary productivity of seagrass beds in Laguna Madre from acute and chronic effects of maintenance dredging, while ensuring navigation capability, is to remove dredged materials from the shallow waters of the ecosystem.

Environmental correlates of the distribution of southern river otters Lontra provocax at different ecological scales

Endangered southern river otters Lontra provocax (huillin) are confined to southern Chile and three isolated areas of Argentina, the most important being the Nahuel Huapi National Park, where distribution surveys were conducted in 1983, 1995 and 2000. We used these survey data together with a Geographical Information System analysis to elucidate the factors that are important in determining huillin distribution at different ecological scales. There were significant heterogeneities in the distribution of huillin between river basins, between habitats, between lakes relative to dispersal routes and topography, and between sample points within a lake. From these analyses three factors emerge as potentially related to huillin distribution, and further elucidation of these are suggested as priorities for research to underpin the species' conservation. These factors are: a) the distribution of crustacean prey, with absence of huillin from one river basin and from mountainous streams correlated with low densities of crustaceans in these places; b) human habitation, which may explain why huillin dispersed southwards, but not northwards, from the main lake, although underwater visibility may also be relevant; c) the presence of the introduced American mink Mustela vison, because although there was a positive relationship between the occurrence of mink and huillin in Lake Nahuel Huapi, there was a negative relationship in their occurrence between other lakes. We discuss possible explanations for all three factors.

Fauna of Natural Seagrass and TransplantedHalodule wrightii(Shoalgrass) Beds in Galveston Bay, Texas

AbstractWe compared nekton and benthos densities and community compositions in a natural mixed seagrass bed dominated byHalodule wrightii(shoalgrass) with those found in three shoalgrass transplant sites and adjoining sand habitats in western Galveston Bay, Texas, U.S.A. Quantitative drop traps and cores were used to compare communities up to seven times over 36 months post‐transplant where transplant beds survived. Total densities of fishes, decapods, annelids, benthic crustaceans, and most dominant species were significantly higher in natural seagrass than in transplanted shoalgrass or sand habitats during most sampling periods. On occasion, fish and decapod densities were significantly higher in transplanted shoalgrass than in adjoining sand habitats. No consistent faunal differences were found among transplant sites before two of three sites failed. Taxonomic comparison of community compositions indicated that nekton and benthos communities in natural seagrass beds were usually distinct from those in transplanted beds or sand habitats, which were similar. We conclude that reestablishing a shoalgrass bed that resembles a natural seagrass bed and its faunal communities in the Galveston Bay system will take longer than 3 years, provided that transplants persist.

Salt marshes as nurseries for nekton: testing hypotheses on density, growth and survival through meta-analysis

We examined the nursery role of salt marshes for transient nekton by searching the lit- erature for data on density, growth, and survival of juvenile fishes and decapod crustaceans in marshes and using meta-analyses to test hypotheses. We analyzed density data from 32 studies con- ducted throughout the world. Based on fish density, habitat types could be ranked from highest to lowest as: seagrass > vegetated marsh edge, nonvegetated marsh, open water, macroalgae, oyster reefs > vegetated inner marsh. However, patterns of habitat use varied among the 29 fish species rep- resented. For decapod crustaceans (seven species), habitat types were ranked: seagrass > vegetated marsh edge > nonvegetated marsh, vegetated inner marsh, open water, macroalgae > oyster reef. We identified only 5 comparative studies on transient nekton growth in salt marshes. Fish growth in non- vegetated salt marsh was not significantly different from growth in open water or in macroalgae beds but was significantly lower than in seagrass. Growth of decapod crustaceans was higher in vegetated marsh than in nonvegetated marsh. Nekton survival in salt marsh (11 studies analyzed) was higher than in open water, lower than in oyster reef/cobble and not significantly different from survival in seagrass. When density, growth and survival are all considered, the relative nursery value of salt marshes for nekton appears higher than open water but lower than seagrass. Vegetated marsh appears to have a higher nursery value than nonvegetated marsh; however, tidal dynamics and nek- ton movement among marsh components complicates these comparisons. The available data have a strong geographical bias; most studies originated in the northern Gulf of Mexico or on the Atlantic coast of the United States. This bias may be significant because there is some evidence that salt marsh nursery value is dependent on geography, salinity regimes and tidal amplitude.

Distribution and abundance of zooplankton in the Waikato River, New Zealand

Potamoplankton is often a well developed component in large lowland rivers, yet little is known about its structure in New Zealand's longest river, the Waikato River. To redress this gap we sampled bimonthly at seven sites along the length of the river over 12 months. Rotifers were the dominant zooplankton in the Waikato River making up 85% of the total densities. Cladocerans represented 9% and copepods only 6%. Rotifers were also the most taxonomically rich group with 41 species in 20 genera identified throughout the study. Thirty rotifer species and nine genera represent new records for the river – two cladoceran species were also recorded for the first time. The highest densities of crustaceans and rotifers were found in the hydro lakes. Densities of crustaceans decreased with increasing distance downstream and densities of rotifers were on average 15 times greater than crustaceans in the lower river. The seasonality of Crustacea was similar to that in New Zealand lakes and rivers with high densities in summer and minimum densities over the winter period. Total rotifer densities showed a similar trend although there were marked seasonal differences between individual species.

Great differences in peracarid crustacean density between the Arctic and Antarctic deep sea

Great differences in peracarid crustacean density between the Arctic and Antarctic deep sea

The influence of nursery ground processes in the determination of year-class strength in juvenile plaice Pleuronectes platessa L. in Port Erin Bay, Irish Sea

The interannual variability in settlement and mortality of juvenile plaice Pleuronectes platessa L. was investigated between 1992 and 1998 on Port Erin Bay, west side of the Isle of Man, Irish Sea. The dampening influence of factors operating on the nursery grounds was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Thus the nursery ground processes were likely to be density dependent. Shrimp and crab densities were low in Port Erin Bay and probably had little predatory impact on young plaice. Crustacean densities were not significantly related to winter temperatures. In the Irish Sea, year-class strength is determined during the nursery ground phase, in contrast to the North Sea where determination of year-class strength occurs prior to the nursery ground phase.

Distribution patterns of benthic crustaceans in a formerly meromictic lake with changing trophic conditions (Lake Piburg, Tyrol, Austria)

The change of the crustacean fauna in the soft sediment of an Alpine lake (Lake Piburg) due to reoligotrophication is investigated. Samples were taken with a Core-Sampler (Kajak-type) along two transects from 5 m to the maximum depth of the lake (24 m) in November 1994 and May 1995, and compared to a study from 20 years ago. The crustacean fauna typically consisted of Cladocera, Copepoda, and Os-tracoda with benthic, planktonic and diapausing representatives. Density and biomass of crustaceans showed high variation with water depth and season. The average density of crustaceans in the soft sediment reached 105,289 ind. m -2 with a biomass of 409.9 mg DW m -2 in November; in May these values were much lower (25,333 ind. m -2 and 66.1 mg DW m -2 ). Comparisons with an earlier study, when the hypolimnion was anoxic, suggest that changes in distribution patterns, densities, and species composition were due to a change in trophic state. The recovery of the hypolimnion reflected a trend towards oligotrophy. In contrast, the crustacean community in the hypolimnion still indicates eutrophy.