Zoobenthic Biomass Research Articles

Effects of cyprinid removal and reintroduction: Diamond Lake, Oregon

Eilers J, Miller R, Loomis D, Vogel A. 2023. Effects of cyprinid removal and reintroduction: Diamond Lake, Oregon. Lake Reserv Manage. 39:156–173. Diamond Lake, located in the Oregon Cascade Range, was treated with rotenone in 2006 to remove invasive populations of cyprinids, Gila bicolor and Notemigonus crysoleucas. The treatment successfully removed all fish, and the lake was restocked in 2007 with rainbow trout (Oncorhynchus mykiss). The treatment resulted in large increases in transparency, large cladocerans (Daphnia pulicaria), and benthic invertebrates. The previous cyanobacterial blooms were comprised almost exclusively of Anabaena [Dolichospermum], whereas the current populations of cyanophytes include Gloeotrichia. Cyprinids were reintroduced into the lake and documented in 2008 (Notemigonus crysoleucas) and 2015 (Gila bicolor), likely contributing to a decline of several metrics of water quality. Piscivorous trout (Salmo trutta and Salmo trutta x Salvelinus fontinalis) were introduced into the lake starting in 2016 to control the introduced cyprinids. The cyprinid populations have stabilized, and most metrics of lake status (Secchi disk transparency, phytoplankton biovolume, abundance of large cladocerans, zoobenthic biomass, trout condition factor) indicate that the lake has improved substantially since the treatment and introduction of piscivorous trout. It is unclear whether the cyprinid populations are constrained by behavioral mechanisms associated with the introduction of the piscivorous trout or whether other factors currently keep the cyprinids in check. The success of this biomanipulation project requires continued monitoring and use of adaptive management strategies to respond to changes in fish composition.

The commonly applied Brey model to estimate benthic secondary production cannot be used for communities dominated by large bivalves

The Brey model to calculate secondary production from data on biomass and mean individual weight is widely used, but so far it has not been tested for communities in which one or a few species dominate. On the base of a long-term (tens of years) data series on estimates of densities, biomass, mean individual weights and annual production of the most important bivalve species (together accounting for about 60% of the zoobenthic biomass) in a vast tidal-flat area, we compared production values obtained by two methods: the weight-increment summation method and the Brey model. In 4 out of the 5 studied bivalve species, the Brey estimates were on average substantially lower than the direct estimates. As a consequence, the Brey method seriously underestimated the production of the entire bivalve community in about half of the years. In 10 out of the 28 years, the Brey estimates were even less than half of the actual bivalve production. The one species (Limecola balthica) with a satisfactory match between the two methods was (as contrasted to the 4 other species: Cerastoderma edule, Mytilus edulis, Mya arenaria, Ensis leei) characterized by low annual weight increments and the simultaneous presence of many year classes, none of them strongly dominating. As secondary production of the studied ecosystem is dominated by the group of large bivalves, Brey estimates for such bivalve-dominated systems are inadequate for a study of their functioning.

Half a century of monitoring macrobenthic animals on tidal flats in the Dutch Wadden Sea

Macrobenthic animals living in a tidal-flat area in the westernmost part of the Wadden Sea were monitored for 50 yr (1970-2019) using consistent methods. About 100 papers were published on this project. We review a number of results and conclusions on observed changes and their possible underlying causal processes. The most significant changes in population sizes and growth rates of several species could be attributed to climate warming (by about 2°C), along with increasing trends in species richness and total late-winter zoobenthic biomass. In the initial years, eutrophication (doubling of nutrients and chlorophyll concentrations) resulted in a doubling of zoobenthic biomass. The subsequent de-eutrophication after the mid-1980s was reflected only in the biomass values observed in late summer. A long-term trend in food supply for birds was not observed. Disturbances from fisheries were intermittent and modest. In several bivalve species, magnitudes of production and biomass were determined primarily by recruitment variation, which was mainly caused by spring abundance of epibenthic predators (shore crabs and shrimps). Their abundance increased with temperatures in the preceding winter. In contrast to this top-down regulation, bottom-up processes apparently played only a minor role in the determination of bivalve biomass. Rarely occurring extremely high bivalve numbers resulted in reduced rates of growth and production. We conclude that the uniquely long monitoring of the tidal-flat macrozoobenthos yielded data series which not only indicated several long-term trends, but also contributed to our insight in processes underlying the observed trends. Most of the observed trends were related to climate change and eutrophication followed by de-eutrophication.

Understanding Environmental Changes in Temperate Coastal Seas: Linking Models of Benthic Fauna to Carbon and Nutrient Fluxes

Coastal seas are highly productive systems, providing an array of ecosystem services to humankind, such as processing of nutrient effluents from land and climate regulation. However, coastal ecosystems are threatened by human-induced pressures such as climate change and eutrophication. In the coastal zone, the fluxes and transformations of nutrients and carbon sustaining coastal ecosystem functions and services are strongly regulated by benthic biological and chemical processes. Thus, to understand and quantify how coastal ecosystems respond to environmental change, mechanistic modelling of benthic biogeochemical processes is required. Here, we discuss the present model capabilities to quantitatively describe how benthic fauna drives nutrient and carbon processing in the coastal zone. There are a multitude of modeling approaches of different complexity, but a thorough mechanistic description of benthic-pelagic processes is still hampered by a fundamental lack of scientific understanding of the diverse interactions between the physical, chemical and biological processes that drive biogeochemical fluxes in the coastal zone. Especially shallow systems with long water residence times are sensitive to the activities of benthic organisms. Hence, including and improving the description of benthic biomass and metabolism in sediment diagenetic as well as ecosystem models for such systems is essential to increase our understanding of their response to environmental changes and the role of coastal sediments in nutrient and carbon cycling. Major challenges and research priorities are (1) to couple the dynamics of zoobenthic biomass and metabolism to sediment reactive-transport in models, (2) to test and validate model formulations against real-world data to better incorporate the context-dependency of processes in heterogeneous coastal areas in models and (3) to capture the role of stochastic events.

Is food supply for shellfish-eating birds in the western Wadden Sea affected by the between-species synchrony in year-to-year fluctuations of bivalve population parameters?

For a better understanding of functioning and stability of ecosystems, it is important to know to what extent constituent species show similarity in their long-term fluctuation patterns, i.e. whether their numbers and biomass frequently show simultaneous peaks and lows. Synchronic peaks and lows of important species would enhance variability in the functioning of the entire system and might affect its stability. When fluctuation patterns of individual species are largely independent, their peaks and lows would tend to extinguish each other’s effect on overall parameters (such as total zoobenthic biomass), thus promoting system stability. A long-term (46 years) monitoring study of the macrozoobenthos in a large (50 km2) tidal-flat area revealed that the 4 most important bivalve species (3 suspension feeders: Cerastoderma edule, Mytilus edulis, Mya arenaria and 1 deposit/suspension feeder: Limecola (Macoma) balthica) frequently showed peak numbers of their recruits in the same years. The annual growth rates of the three suspension feeding species showed some synchrony as well. Annual survival rates, on the other hand, did not show any synchronization, wiping out the initial synchrony of numbers within less than 2 years. As a result, annual biomass values did not show any positive between-species correlations. Annual amounts of bivalves that are accessible as bird food rarely declined to levels below 5 g AFDW m−2 and showed limited (5 to 10 fold difference between maximal and minimal values) variation. Oystercatchers left the area quicker in late winter and showed increased death rates at very low levels of food supply. Total bivalve bird food did not show any significant long-term trend. However, biomass of Mya arenaria showed an increasing trend and that of Limecola balthica a declining trend.

Macrobenthic communities at high southern latitudes: Food supply for long‐distance migratory shorebirds

AbstractIntertidal soft‐bottom assemblages located at high latitudes provide a critical food source for long‐distance migratory animals which link biodiversity across distant areas. On the southern Pacific coasts of South America, however, comprehensive information about macrobenthic assemblages at these habitats is lacking. Here we provide an inter‐annual estimation of food supply at the southern limit of the Pacific temperate zone within a Site of Hemispheric Importance for the conservation of Arctic breeding shorebird populations during the nonbreeding season. Macrobenthic communities on Isla Grande of Chiloé are dominated by Polychaetes, Bivalves and Malacostraca. Average biomass (4.4–9.6 g ash‐free dry weight m−2) falls within the values reported for temperate intertidal areas located at around 40°S latitude. While total available biomass within each bay was similar during the three sampling years, the annual contribution of each class varied. The major contributor to zoobenthic biomass (Polychaetes) was indeed one of the main preys for the most abundant shorebird species at Chiloé. Arctic breeding migratory shorebird populations seem thus to encounter a predictable and abundant food supply at high southern latitudes on Isla Grande of Chiloé.

Mortality of post-settlement clams Rangia cuneata (Mactridae, Bivalvia) at an early stage of invasion in the Vistula Lagoon (South Baltic) due to biotic and abiotic factors

The clam Rangia cuneata, originating from the Gulf of Mexico, was recorded in the Vistula Lagoon for the first time in the early 2010s, and quickly became the dominant component of the zoobenthic biomass. To assess mortality as a factor potentially controlling the growth of Rangia population, a year-long field experiment involving marked bivalves placed in sediment-filled trays deployed on the bottom was conducted in 2014 and 2015. Predator-induced mortality of the clams was low in summer, and very high in the winter–spring period. It was inversely proportional to the size of the clams. Such changes can be partially attributed to predation from at least five fish and three duck species, which contained clams in their digestive tracts. Non-predatory mortality particularly affected large individuals, and was highest in spring, several weeks after the end of winter. We hypothesize that it could be caused by persistent low temperatures over several winter months which led to considerable weakening of the condition of clams. A long winter could also reduce their resistance to environmental stress and potential effect of epibionts, as well as increase susceptibility to predation.

Invertebrate food sources for waterbirds provided by the reconstructed wetland of Nyirkai-Hany, northwestern Hungary

The Nyirkai-Hany wetland reconstruction area in northwestern Hungary is now designated as a Ramsar and a Natura 2000 site. It was created in 2001–2002 by the Fertő-Hansag National Park Directorate to restore a part of the formerly drained large wetland called Hansag and to offer waterbirds a suitable habitat for feeding and breeding. We focused on this aim of the restoration project and studied the temporal and spatial variation in abundance of birds and their invertebrate prey in this newly created wetland. From April 2007 until May 2008, we sampled plankton, nekton and benthos of different habitats monthly and monitored waterbirds weekly on the three different areas of the Nyirkai-Hany. During our investigations, 135 invertebrate and 53 waterbird species were recorded. Benthos and macrophyte decomposition accelerating guilds were the most abundant waterbird guilds—besides the dominant grazing importer material transporter guild, represented primarily by geese—in the Nyirkai-Hany. Zooplankton assemblages primarily consisted of small species not easily used as a food by planktivorous waterbirds. The low density of zoobenthic biomass and the small extent of shallow water mudflats probably accounted for the scarcity of the bioturbing guild group of birds. Nektonic biomass varied greatly among locations having different vegetation types, was greatest in the shallow water areas dominated by Typha, Carex and Phragmites species and lowest at offshore vegetation-free sites. Chironomids, mayflies and odonates were especially abundant and their biomass significantly correlated with several waterbird species, mainly belonging to the macrophyte decomposition accelerating guild (e.g. Anas platyrynchos, Fulica atra). This guild itself, which has increased in abundance in recent years, showed an exceptionally strong correlation with odonate abundance. These results indicate the growing importance of the Nyirkai-Hany wetland area as a foraging site for waterbirds.

What a difference a species makes: a meta–analysis of dreissenid mussel impacts on freshwater ecosystems

We performed a meta‐analysis of published studies and long‐term monitoring data sets to evaluate the effects of dreissenid mussels (Dreissena polymorpha and D. rostriformis bugensis), two of the world's most problematic biological invaders, on the biogeochemistry, flora, and fauna of lakes and rivers across North America and Eurasia. Dreissenid effects were structured along two distinct energy pathways. For the pelagic–profundal pathway, large mean reductions in phytoplankton (−35% to −78%) and zooplankton (−40% to −77%) biomass occurred and were dependent on habitat type. The largest effects were found in rivers, followed by littoral and pelagic habitats in lakes. In contrast, benthic energy pathways within littoral habitats of lakes and rivers showed dramatic increases in mean benthic algal and macrophyte biomass (+170% to +180%), sediment‐associated bacteria (about +2000%), non‐dreissenid zoobenthic biomass (+160% to +210%), and total zoobenthic biomass, which includes dreissenid mussel soft tissues (+2000%). Our study quantifies the remarkable ability of these invasive mussels to shift aquatic food webs and energy flow from pelagic–profundal to benthic–littoral energy pathways, and it provides a basis for forecasting their impacts in diverse freshwater ecosystems. Our meta‐analysis approach was a powerful tool for moving beyond the idiosyncrasies of individual case studies and may be equally powerful for assessing impacts of other biological invaders.

Ten years trends in the oligochaete and chironomid fauna of Lake Neuchâtel (Switzerland)

Quantitative surveys of benthic macroinvertebrates (oligochaetes and chironomids) were conducted during 1992, 1997, 2000 and 2002 at a depth of 40 m to monitor the biological quality of sediments in Lake Neuchâtel. Recent declines in frequency of occurrence and abundance of oligochaete species characteristic of oligotrophic conditions (Stylodrilus heringianus, Embolocephalus velutinus) contrasted with the improvement of water quality metrics. Total phosphorus concentrations in lake waters decreased from 63 mg m - 3 in 1980 to 10 mg m - 3 in 2002. Since 1992, significant reductions of total zoobenthic biomass have been recorded and the chironomid community structure reflects typical oligomesotrophic conditions in the upper sediment layer. The population of the oligochaete species Potamothrix vejdovskyi is in clear expansion and indicates that this new species for the lake (1986) has found good conditions for a successful colonization. These divergent responses within the zoobenthic community are discussed according to three main hypotheses: implications of toxic pollutants like heavy metals and organic micropollutants in the sediment, impact of algae on oxygen conditions on the bottom layer and different biological responses of oligochaetes to competition for food and space.

Zoobenthic biomass limited by phytoplankton abundance: evidence from parallel changes in two long-term data series in the Wadden Sea

We address the question of whether year-to-year variability in pelagic algal food supply can explain long-term variability in macrozoobenthic biomass in an estuarine area. Starting in the early 1970s, quantitative data were frequently collected in standardized ways in the western part of the Dutch Wadden Sea on (1) concentrations of phytoplankton species and chlorophyll (and rates of primary production) in the main tidal inlet (Marsdiep) and (2) numerical densities and biomass of macrozoobenthic animals (and growth rates in a few species) in a nearby extensive tidal-flat area (Balgzand). In both data series, the most distinctive feature was a sudden change that took place around 1980, viz. a rather sudden and persisting doubling of concentrations of chlorophyll and algal cells and of primary production rates, as well as of numerical densities and biomass of zoobenthos. From these parallel changes we hypothesise that algal food largely determines the abundance of zoobenthos in the Wadden Sea. The following observations substantiate this hypothesis: (1) the significant correlation between annual mean values of chlorophyll concentration and overall mean numerical density and biomass of zoobenthos (as estimated after an appropriate time lag), (2) the observed limitation of zoobenthic biomass doubling (after the doubling of food supply) to areas with already high biomass values (where food demand was high and food could therefore be in short supply), (3) the limitation of a strong response to changes in food supply to functional groups that are directly dependent on algal food, i.e. suspension and deposit feeders, as opposed to carnivores, (4) the significant correlation between annual growth rates in Macoma balthica and food supply in the growing season, particularly in areas close to the tidal inlet where food concentrations were monitored. Some other factors were identified that could decisively influence zoobenthic abundance locally and/or temporarily. Harsh environmental conditions will have limited zoobenthic biomass in extreme areas such as the upper part of the intertidal and areas exposed to strong currents and wave action. Severe winters temporarily reduced the abundance of several sensitive (southern) species, including most of the infaunal and epifaunal predators. Even stronger temporary and local reductions of zoobenthic biomass occurred as a consequence of fishery activities for such benthic species as cockles, mussels and lugworms. Recovery after temporary biomass reductions was generally rapid, but late-winter values of total-Balgzand zoobenthos biomass never exceeded an upper level of 45 g AFDW per m 2 probably set by maximal annual rates of primary production of between 400 and 500 g C per m 2.

Hydrobiological effects of gathering reindeer at an arctic lake in Russia

For the separation of reindeer, animals are collected on the shore of lake Layato (67 60'N, 56 E) about one month in every summer. Feces and urine produced by the herd are washed directly into the lake, which results in changes in the water quality and lake biota. When for instance in 1991 the herd numbered about 1000 animals, ammonium concentrations and zoobenthic biomass increased markedly after the period of separation.

Synchronized reproductive success in the main bivalve species in the Wadden Sea: causes and consequences

This paper surveys data obtained during 3 decades (1969-1999) of monitoring the macrobenthic fauna of a 50 km 2 tidal-flat area in the Wadden Sea (Balgzand) and compares the annual figures with similar long-term data series from other parts of the Wadden Sea (Groningen, Norderney). Despite enormous year-to-year variability in annual recruitment of particularly the bivalves, total-biomass values were remarkably stable. Detailed data are presented on the annual variability in recruitment success of the 4 most important bivalve species of the Balgzand area (Cerastoderma edule, Macoma balthica, Mya arenaria, and Mytilus edulis). Together these 4 species usually accounted for more than half of the total zoobenthic biomass and largely governed its year-to-year fluctuations. Recruit numbers of the 4 species observed in 27 summers showed similar relationships with the character of the foregoing winter. Such relationships were also observed in other Wadden Sea areas in The Netherlands and Germany. These common relationships resulted in a certain synchrony of recruitment success between the species (within areas) and between areas (within species). Thus, over vast areas (hundreds of km), bivalve recruit densities were generally (though not invariably) high after severe winters and low after mild winters. The resulting strong year-to-year variability in recruit numbers (with standard errors exceeding long-term means) was only just passed on to subsequent total-biomass values for the following 3 reasons: (1) the biomass of any species was generally composed of more than 1 year class, (2) the various species did not reach their maximal biomass at the same age (time elapsed after recruitment), and (3) feedback mechanisms (enhanced reduction of numbers at high abundance) were effective in some species.

Local differences in macrozoobenthic response to enhanced food supply caused by mild eutrophication in a Wadden Sea area. Food is only locally a limiting factor

In the late 1970s food supply to herbivores suddenly doubled in the western half of the Dutch Wadden Sea probably as a consequence of increased nutrient concentrations. The response of zoobenthos was studied during 1970–1990 as part of a long‐term monitoring program, including annual sampling at 15 fixed stations with very different environmental conditions (elevation, sediment grain size) on Balgtand, a 50‐km2 tidal flat area. The total zoobenthic stock in the affected area roughly doubled within 2 yr after the doubling of the stock of planktonic algae. However, the magnitude of the response differed from station to station. We tested the hypothesis that a strong positive response would occur particularly at stations where food could be limiting, i.e. where zoobenthic food demand was high relative to food supply. Biomass increases of zoobenthos in the order of a doubling were indeed restricted to the part of Balgzand characterized already in the 1970s by high biomass values. The zoobenthic biomass hardly increased in two areas of Balgzand with relatively low values for zoobenthic biomass and species richness. In these impoverished areas environmental conditions are harsh and the benthic communities are dominated by small opportunistic species. One of these areas is characterized by a high intertidal elevation (short daily inundation periods) combined with a sheltered location near the coast (resulting in muddy sediments), the other by severe exposure to currents and wind (rasultinh in mobile sands with silt and organic‐matter contents) due to its offshore position. We conclude that food limitation only existed in the midshore communities living under relatively mild abiotic conditions. Apparently, the benthic consumer populations benefitted from eutrophication‐induced enhancement of their food supply by enlargement of their stocks only where they were not too stressed by unfavorable abiotic conditions.

Wave Velocity Thresholds for Fine Sediment Accumulation in Lakes, and Their Effect on Zoobenthic Biomass and Composition

Deposition of fine sediments occurs at depths where water velocity at the bottom is less than the threshold velocity required to keep the particles in suspension. In this paper we demonstrate that the depositional boundary depth (DBD) can be predicted from models based on wave height and lake morphometry, and that such depositional boundaries greatly affect the biomass and composition of the zoobenthos. Lake Memphremagog was chosen for this study since it exhibits a great deal of variation in exposure and bottom slope, which make the DBD highly variable from place to place. In spite of this variation, our model predicted its location very accurately even along transects with highly complex topography. Sites that our models predicted would be depositional, always had fine sediments and had more than twice the zoobenthic biomass of non-depositional sites, which had sediments made up of silt or silty-sand. Epilimnetic sites had more than twice the average biomass of hypolimnetic sites of the same depositiona...

Consequences of the Sudden Removal of Nearly All Mussels and Cockles from the Dutch Wadden Sea

Abstract. Three successive years of recruitment failure, intensive fishery, and high rates of natural mortality eliminated all intertidal mussel beds and nearly all cockle beds from the Dutch Wadden Sea in the course of 1990. In late 1990 and early 1991, zoobenthic biomass and particularly bivalve stocks were unprecedentedly low. The following unusual events were noted: Relatively high diatom and chlorophyll concentrations in the autumn of 1990 and an exceptionally early and dense spring bloom of diatoms in March 1991; Exceptionally early onset of rapid growth in the bivalve Macoma balthica in March 1991; Above‐average weights of the soft parts of bivalves in the 1990/91 winter; Elevated mortality rates in bivalves that could serve as alternative prey for birds specialised on big bivalves (oystercatchers and eiders); Signs of food shortage in such birds: high mortality rates and emigration to other feeding areas; A rapid recovery of the benthos started in the summer of 1991, accelerated by an exceptionally high recruitment in some of the affected species. It is concluded that the Wadden Sea ecosystem responded in an elastic way to the removal of two of its key species. The events are not seen as merely coincidental but are regarded as a chain of causes and effects.

Sublittoral macrozoobenthic assemblages in the enclosed sediment-polluted Rhine-Meuse Delta; their relationship to environmental conditions

Composition, densities and biomass of sublittoral macrozoobenthos were investigated at 14 sites in the enclosed Rhine-Meuse Delta. The aim was to identify the main macrozoobenthic assemblages and to establish their relationship with environmental conditions, including sediment pollution. Most sites were dominated by the tubificidsLimnodrilus hoffmeisteri andL. claparedeanus. Four zoobenthic assemblages could be identified. They are related to different geomorphological phases of a basin filling up with fluvial sediment causing a longitudinal sediment gradient. The ’Rhine river’ assemblage was found in the Nieuwe Merwede, filled up in the 1970's, the ’soft mud’ assemblage in the Hollands Diep at sites with high sedimentation rates; the ’basin silt’ assemblage in the Amer, Hollands Diep and Haringvliet, at sites with lower sedimentation rates, and the ’basin shoal’ assemblage at former flats with low sedimentation rates. CANOCO was used to relate the TWINSPAN indicator species to a selection of environmental parameters. Sediment pH, water content, sand fraction, total polycyclic aromatic hydrocarbons (PAH) and current velocity explained 44% of the variance of the average abundance of the indicator taxa per site. Sediment firmness, indicated by sediment water content, and high ammonium concentrations probably were together responsible for the very low densities in the ’soft mud’ assemblage. Densities and biomasses in the other assemblages were low as well, except for theDreissena polymorpha habitat, where very high total zoobenthic biomass values (max. 55.8 g m−2) were observed. It is argued that sediment contamination has a considerable impact on macrozoobenthic densities and species composition in all assemblages, the ’basin shoal’ assemblage excepted.

Long-term effects of mechanical harvesting of lugworms Arenicola marina on the zoobenthic community of a tidal flat in the Wadden Sea

More than half of the annual catch of about 30 million lugworms Arenicola marina from the Dutch Wadden Sea originates from digging machines which make 40-cm deep guilles in a few restricted tidal-flat areas (Texel, Balgzand) in the westernmost part of the Wadden Sea. Four successive years (1978–1982) of frequent disturbance by a lugworm dredge of one of the 15 sampling stations involved in a long-term study of the dynamics of the macrozoobenthos on Balgzand allowed a study of long-term effects of mechanical lugworm digging. Within an area of about 1 km 2, a near-doubling of the annual lugworm mortality rate resulted in a gradual and substantial decline of the local lugworm stock from more than twice the overall Balgzand mean at the start of the 4-year digging period to a value close to this mean at the end of the period (when the dredge moved to a richer area). Simultaneously, total zoobenthic biomass declined even more by the almost complete extinction of the population of larger gaper clams Mya arenaria that initially comprised half of the total biomass. Of the other, mostly short-lived, species only Heteromastus filiformis showed a clear reduction during the dredging period. Recovery of the biomass of the benthos took several years, particularly by the slow re-establishment of a Mya population with a normal size and age structure.

Evaluating Wader Habitats in Roebuck Bay (North-western Australia) as a Springboard for Northbound Migration in Waders, with a Focus on Great Knots

We studied the diet and foraging behaviour of the Great Knot Calidris tenuirostris and to lesser extent of the Red Knot Calidris canutus rogersi and other waders at Roebuck Bay, Broome, Western Australia, in March-May 1991, the period of northward migration. Visual feeding observations, benthic fauna sampling and faecal analysis showed that the diet of Great Knots consisted mainly of bivalves. The total zoobenthic biomass in Roebuck Bay averaged 13.9 g ash free dry mass (AFDM) m-2, of which 5.7 and 6.0 g AFDM m-2 were available (sensu Zwarts et al. 1992) to Red Knots and Great Knots, respectively. The profitabilities of several possible prey types are discussed and compared with the recorded diet. Intake rates of Great Knots were estimated on the basis of recorded prey intake (0.24 mg AFDM s-1) and defaecation rates (0.33 mg AFDM s-1), and used to calculate mass gains prior to northward migration. The repercussions of the large tidal range on foraging possibilities are discussed. Changes in tidal regime may mean that Great Knots do not always have access to their preferred feeding sites. We indicate that the array of prey types available to, and taken by, Great Knots is much greater in Roebuck Bay than in the Wadden Sea. We discuss the repercussions of diet diversity on wader distribution. When compared with other springboard areas such as the Banc d'Arguin in Mauritania and the Wadden Sea in western Europe, the available Roebuck Bay benthic food stocks are larger.

The role of chironomids (Diptera) in the profundal macrozoobenthos in Finnish reservoirs

The proundal macrozoobenthos of 12 Finnish inland polyhumic reservoirs was studied by quantitative sampling in 1984 and 1986. Additionally, the succession of the macrozoobenthos in one young reservoir (Lake Kyrkosjarvi) was studied during its first six years. The zoobenthos material was analyzed using the DCA (detrended correspondence analysis) method, and the ordination of sites (i.e. reservoirs), was related to associated environmental data. The role of chironomids was, in general, important in the profundal of the reservoirs. Because of the coarse bottom quality at many sampling sites, many littoral taxa were found in the profundal zone. The genusChironomus dominated (>50% of the total zoobenthic biomass) especially in the acidic, upper-reach reservoirs.Chironomus larvae also dominated in the zoobenthos of Lake Kyrkosjarvi during its first three years. Some of the reservoirs showed similarities with the dystrophic lake type of the lake typology system (Brundin, 1956). Because of the initially eutrophic character of the bottom, many reservoirs belonged to eutrophic lake types. Finnish reservoirs appear to contain a zoobenthic fauna intermediate between natural lakes and bog pools. This theme was discussed from the ecological and evolutionary point of view ofChironomus species.