Predation by juvenile Platichthys flesus (L.) on shelled prey species in a bare sand and a drift algae habitat

Due to increasing eutrophication of the coastal Baltic waters, drifting algae are a common phenomenon. Drifting algal mats accumulate on shallow sandy bottoms in late summer and autumn, and affect the ambient fauna. Juvenile flounder, Platichthys flesus, utilize these habitats during their first few years. They feed on benthic meio- and macrofauna; part of their diet consists of shelled species, such as Ostracods, and juvenile Hydrobia spp. and Macoma balthica. Earlier studies have shown that up to 75% of ostracods and 92% of hydrobiids survive the gut passage of juvenile flounder, while all M. balthica are digested by the fish. We conducted laboratory experiments to study how the shelled prey responded to a drift algal mat, and the predation efficiency of juvenile P. flesus on these prey species on bare sand and with drifting algae (50% coverage). Hydrobia spp. utilized the drift algae as a habitat and, after 1 h, 50% had moved into the algae; ostracods and M. balthica were more stationary and, after 96 h, only 23 and 12%, respectively, were found in the algae. For the predation efficiency of P. flesus, a two-way ANOVA with habitat (algae, bare sand) and predation (fish, no fish) as factors revealed that both algae and predation affected negatively the survival of all three prey species. The algae, thus, affected the predation efficiency of juvenile P. flesus and the consumption of prey was much reduced in the algal treatments compared to the bare sand. This was due probably to increased habitat complexity and the ability of prey, especially hydrobiids, to use the algal mat as a refuge. Altered habitat structure due to drift algae, together with the resultant changes in habitat (refuge) value for different prey species, may profoundly change the structure of benthic communities.     

Immediate Effects of Experimental Human Trampling on Mid-Upper Intertidal benthic Invertebrates at the Asinara Island MPA (NW Mediterranean)

To assess potential risks of human visitation to ecological communities, the immediate effects of human trampling were investigated experimentally on small invertebrates inhabiting mid-upper intertidal hard bottoms covered by algae. Two different experimental intensities of trampling (60 and 120 footsteps) and controls (with no trampling) were applied to quadrats 20×20 cm in size (experimental area), within the two ‘no-entry, no-take’ zones of the Asinara Island MPA (Italy, Mediterranean Sea). One day after trampling ended, samples of benthic fauna were collected and the animals attributed to macrofaunal and meiofaunal components. Analyses of variance on the nine most common taxa of macrofauna identified significant higher abundance of bivalves, gammarid amphipods, polychaetes, isopods, oligochaetes in controls than in trampled plots. For nematodes, polychaetes, ostracods, oligochaetes, bivalves, acari, caprellid amphipods and tanaids a significant higher abundance of meiofaunal animals was found in controls than in trampled areas. Although no information on recovery is available, these results suggest that macrofaunal and meiofaunal taxa are vulnerable to this type of disturbance.     

Effects of depth, sediment and grazers on the degradation of drifting filamentous algae (Cladophora glomerata and Pilayella littoralis)

Eutrophication in the northern Baltic Sea promotes growth of annual filamentous algae. The algae detach, gather at the bottom and give rise to algal mats of varying size, density, composition and condition. Dense mats of filamentous algae induce anoxia, which in turn leads to faunal mortality. By a set of field experiments, we have studied the fate of the abundant Cladophora glomerata after detaching from the rocky substrate, and the effect of water depth and sediment on its decay. Further, we have studied the importance of common mesograzers (Gammarus and Idotea) on the rate of degradation of C. glomerata and Pilayella littoralis.Our results show that loose algae at shallow sites (8 m) decompose faster than algae in deeper (18 m) areas. Drifting C. glomerata on the sediment is more rapidly broken down and dissolved than algae floating in the water column, which depends on higher microbiological activity. Dominant amphipods (Gammarus spp) colonise near-shore drift algae quickly, and juvenile bivalves (Cerastoderma glaucum) utilise algae in the water column for settling. Moderate natural densities of grazers (Gammarus spp and Idotea baltica) in the drifting algae did not increase the degradation rates of C. glomerata and P. littoralis. C. glomerata was completely decomposed in 4 months.Our experiments demonstrate the effects of position (depth, water/sediment) and grazing on the degradation of drifting filamentous algae. Mass developments of opportunistic algae occur annually in the study area, and information on the destiny of detached drift algae may help us to predict their longevity and the damage they cause, and hence, to decide on long-term measures needed to improve environmental conditions.     

Meiofauna sediment relations in leeward slope turf algae of Heron Island reef

As part of studies investigating the influence of grazers on reef meiofauna, we assessed the density, composition and richness of meiofauna (retained on a 100-μm sieve) on the leeward reef slope of Heron Reef, GBR, Australia using an airlift vacuum sampling device. Estimates of meiofauna densities ranged between 40 individuals 10 cm⁻² and 290 individuals 10 cm⁻², which is considerably lower than many estimates from carbonate sediments and hard substrates from other reefs and marine habitats. The 17 taxa of meiofauna were dominated by harpacticoid copepods (40%) and nematodes (32%). Varying sediment load within algal turfs explained 37% of variation of meiofauna density. A model is proposed in which increased shelter afforded by high living coral cover reduces meiofaunal losses from grazing and increases sediment loads, balanced by areas of low coral cover in which sedimentation rates are lower and grazing rates higher. At none of the four sites did major differences in abundance occur between November and March sampling events. Together these observations suggest that epilithic meiofaunal communities are generally spatially and temporally predictable at small scales in this reef system, indicating that their ecological services are similarly conservative. Handling editor: I. Nagelkerken     

Distribution and abundance of soft-sediment meiofauna and a predatory goby in a coral reef lagoon

Spatial patterns in the abundance of the softsediment meiofauna and a predatory goby, Valenciennia longipinnis, were examined in the lagoon of One Tree Reef (Great Barrier Reef). The study provided a quantitative framework to assess the importance of physical factors on and predator prey interactions between the meiofauna and V.longipinnis. Patterns of abundance were examined at two spatial scales: among four habitats (100's of m apart) and among sites (10's of m apart) within habitats. Of the four major constituents of the meiofauna (harpacticoid copepods, nematodes, polychaetes and oligochaetes), gut analyses showed that harpacticoid copepods were the primary prey of V.longipinnis. Spatial patterns of meiofaunal abundance in the lagoon were taxon specific. Polychaetes and harpacticoid copepods exhibited significant differences among habitats. Within habitats, however, polychaetes exhibited significant differences between sites whereas copepods were uniformly distributed. Abundances of nematodes and oligochaetes did not differ between habitats. Densities of nematodes differed significantly between sites while the number of oligochaetes were similar at both spatial scales. V.longipinnis was more abundant in shallow habitats than in deep ones. This study suggests that sediment type may be an important factor influencing the distribution of both the goby and the meiofauna. V.longipinnis and two of the four meiofaunal taxa (harpacticoid copepods and polychaetes) were more abundant in the shallow habitat with fine-grained sediments. There was no significant difference between abundances of meiofaunal taxa in sites where V.longipinnis was present or absent. Overall, more fish occurred in the habitat which had the highest densities of harpacticoid copepods.     

A method for rapid abundance estimation of semiplanktonic meiofauna

Many meiofaunal copepods and plathelminths enter the tidal waters at night thus exhibiting a life-style intermediate between benthic and planktonic. At the same time, ostracods may leave their interstitial dwelling and move across the sediment surface. In laboratory experiments, the percentage of plathelminth populations emerging from the sediment varied with the species, temperature, light conditions, and the dimensions of the sediment cores studied, but not with tidal level, season, ambient density of conspecifics, or the sediment composition. Therefore, the swimming activity may be utilised for extraction of semiplanktonic meiofauna provided that the extraction procedure is standardised with respect to temperature, light and core size. For free-living plathelminths from the Wadden Sea intertidal a robust standard procedure is as follows: sediment cores 1.6 cm in diameter (2 cm² surface area) and 3 cm deep are fitted into cylindrical containers and submerged into aquaria containing filtered seawater (ambient salinity, room temperature, darkness) for 24 h. The sediment containers are then removed and the aquarian water filtered through appropriate meshes; the residue contains the emergent faunal component. For plathelminths, this procedure reduces sorting time by some 90% compared with the standard shaking–decantation method and thus makes it possible to process a high number of samples in a short time. Similar procedures may be developed for copepods and epibenthic ostracods. Received in revised form: 28 August 2000 Electronic Publication     

Algae, macrofaunal assemblages and temperature: a quantitative approach to intertidal ecosystems of Iceland

Algae and the associated macrofauna in two Icelandic intertidal ecosystems under cold and warm influence, respectively, were studied with respect to algae-macrofauna relationships and a possible effect of temperature on community structure. Two sites in Iceland were selected, Sandgerdi ligthhouse (64°8′N 22°40′W) on the southwestern coast, and Grimsey Island (66°33′N 18°04′W), in the north, on the Arctic Circle, where sea temperature is considerably lower (5° approximately). The biomass of algae and the number of species of algae and macrofauna were higher in Sandgerdi than in Grimsey, and the patterns of diversity, evenness, biomass and abundance also differed between the sites. In the intertidal zone of Sandgerdi, a total of 28 species of algae and 45 species of macrofauna were identified whereas only 16 algal species and 27 macrofaunal species were found in Grimsey. Canonical correspondence analysis (CCA) using algal biomass as the environmental variable were conducted, and revealed significant relationships between algae composition and the associated macrofauna; some macrofauna taxa showed specific trophic or refuge relationships with algal species. According to the CCA, Corallina officinalis showed the highest correlation with macrofaunal assemblages in both study sites. However, correlations between macrofauna and other algae differed between Grimsey and Sandgerdi. The present study, together with additional observations in Greenland waters, shows a general decrease of species richness and diversity towards the north which may primarily be due to the temperature regime.     

Abundance and composition of the sea-ice meiofauna in off-shore pack ice of the Beaufort Gyre in summer 2002 and 2003

We studied the abundance, biomass and potential ingestion rates of meiofauna in multi-year sea ice (MYI) of the Beaufort Gyre during two icebreaker expeditions in summers 2002 and 2003. Ice cores were taken at a total of ten stations and analyzed for ice temperature, salinity, chlorophyll a (Chl a), and ice meiofauna abundances. In 2002, ice was free of snow and covered with melt ponds. In 2003, snow still covered the ice and a slush-layer was found in the ice-water interface. The vertical distribution of Chl a mostly followed C-shaped curves with elevated concentrations at the bottom and top of the ice. Ice meiofauna was mainly restricted to the bottom 10 cm of the ice and was dominated by turbellarians, harpacticoid copepods and nematodes. The meiofauna abundances (range: 8–3,000 individuals m^–2) and Chl a concentration (range: 0.1–1.7 mg Chl a m^–2) were similar to estimates for MYI of the Transpolar Drift, but about 2 orders of magnitude below coastal fast first-year ice estimates. Calculated potential meiofaunal ingestion rate, based on allometric equations and volume estimates from the literature, was about 1% of published daily algal production rates and was thus unlikely to constrain algal biomass accumulation.     

Intertidal meiofaunal biodiversity with respect to different algal habitats: a test using phytal ostracodes from Southern California

Rocky intertidal algae harbor a diverse invertebrate meiofauna of arthropods, nematodes and other invertebrates. Despite its ecological importance, relatively little is known about the diversity and composition of this important component of intertidal biodiversity. In this study, we quantified species composition, abundance and distribution of ostracodes, an important group of phytal meiofauna, at two different intertidal areas in southern California. In total, we recovered 22 ostracode species from three different orders (16 podocopids, five myodocopids and one platycopid), nearly a quarter of which could not be assigned to existing taxa. The abundance of ostracodes differed significantly among algal types, with structurally complex algae bearing many more ostracodes per gram of algae than simple forms (blade-like algae and the surfgrass Phyllospadix). Although most ostracode species were recovered from multiple kinds of algae, different algae harbored distinct assemblages that could be discriminated statistically on the basis of relative abundances of ostracode species. This segregation of the ostracode fauna according to algal species is evident even over very short spatial scales (<1 m). Finally, ostracode samples from turf-forming algae were more species rich than samples from other kinds of macroalgae. Since turf-forming algae are easily damaged by human trampling, this component of ostracode biodiversity may be particularly vulnerable to anthropogenic impacts on the intertidal habitat. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: K. Martens     

Drifting algal mats as an alternative habitat for benthic invertebrates: Species specific responses to a transient resource

Patchy occurrences of benthic drift algae (i.e. loose lying macroalgal mats) may increase habitat complexity on normally bare soft bottoms, but at the same time, extensive amounts of drifting algal mats are known to stress the benthic fauna. This paper presents results of the first detailed study of the fauna associated with drift algal mats in the northern Baltic Sea. In order to assess the importance of drifting algae as an alternative habitat for benthic fauna, benthic drift algal mats were sampled on shallow (2-9 m) sandy soft bottoms in the outer archipelago of the ?land Islands (Finland). Species composition, abundance and biomass of the macrofauna associated with algal mats were recorded. The results show that drifting algae at times can harbour very high abundances of invertebrates (up to 1116 individuals/g algal dryweight), surpassing invertebrate densities recorded in seagrass communities. The algal fauna varied between sites and over time, and factors such as ambient benthic fauna, exposure to wind-wave disturbance, depth, and algal coverage and condition influenced the invertebrate community composition of the algal mats. Abundance increased while individual biomass of the animals decreased over time (summer season; July-October). A series of laboratory experiments were conducted in order to test the ability of a few important benthic species to move up into, and survive in a drifting algal mat. Macoma balthica, Hydrobia spp., Nereis diversicolor and Bathyporeia pilosa were used in the experiments, and significant differences in their survival and mobility within drifting algae were recorded. This study shows that benthic species differ significantly in their ability to utilise the algal mats, with mainly opportunistic and mobile taxa such as Hydrobia spp., Chironomidae and Ostracoda benefiting from the algae, whereas infaunal species such as M. balthica and B. pilosa are negatively affected. The occurrence of eutrophication induced drifting macroalgal mats has increased significantly during the last decade in the northern Baltic Sea. Hence, the importance of drifting algae as a stress factor and as an alternative habitat for benthic fauna increases.     

The meiobenthos of five mangrove vegetation types in Gazi Bay,Kenya

The vertical distribution of meiofauna in the sediments ofAvicennia marina,Bruguiera gymnorrhiza,Ceriops tagal,Rhizophora mucronata andSonneratia alba at Gazi Bay (Kenya), is described. Seventeen taxa were observed, with highest densities in the sediments ofBruguiera (6707 ind. 10 cm^–2), followed byRhizophora (3998 ind. 10 cm^–2),Avicennia (3442 ind. 10 cm^–2),Sonneratia (2889 ind. 10 cm^–2) andCeriops (1976 ind. 10 cm^–2). Nematodes accounted for up to 95% of total densities; other common taxa were copepods, turbellarians, oligochaetes, polychaetes, ostracods and rotifers. High densities occurred to about 20 cm depth in the sediment. EspeciallyCeriops sediments show still high densities of nematodes (342 ind. 10 cm^–2) and copepods (11 ind. 10 cm^–2) in the deepest layer (15–22 cm). Particle size and oxygen conditions were major factors influencing meiobenthic distribution;Uca burrows had a major impact on distribution and abundance of meiofauna.     

Distribution,abundance and ecology of the meiofauna in a tropical estuary along the west coast of India

The distribution and abundance of subtidal meiofauna in Mandovi estuary of Goa were studied from June 1983 to June 1984. Monthly faunal abundance ranged from 491 to 2791/10 cm² and dry weight biomass from 0.16 to 2.80 mg 10 cm². Free living nematodes were the dominant group contributing over 75% of the total density and 30 to 42% of the total biomass. Among nematodes the deposit feeders were more abundant in fine muddy substratum while epigrowth feeders dominated in sandy substratum.Harpacticoids were next, comprising 6.9 to 8.7% of the total meiofauna number, followed by turbellaria (3.8–4.5%), polychaeta (2.8–3.2%) and ostracods (1.6–4.5%) The contribution of other groups to faunal density was 4.5–6.2%. In the biomass the ostracods contributed most (29.8–54.7%), followed by nematodes (23.8–34.6%). Over 60% of the fauna occurred in the top 2 cm of the sediment and the faunal density reduced significantly with increasing depth in the sediment. The vertical distribution of meiofauna was positively correlated to the vertical distribution of Eh, chlorophyll a and interstitial water. Seasonality was greatly influenced by the south-west monsoon and the fauna quickly repopulated after the monsoon. Salinity, temperature and food influenced the faunal abundance.     

Effects of flow on meiofauna colonization in artificial streams and reference sites within the Illinois River, Arkansas

Despite frequent disturbances from flow, stream meiofauna form diverse and abundant assemblages suggesting that they are resistant and/or resilient to flow disturbances. Stream flow profoundly influences benthic invertebrate communities but these effects remain poorly understood. We examined the influence of flow on meiofauna colonization at small spatial scales (2–3 m) using artificial streams in conjunction with similar sites (flow, depth, substrates) in the reference stream (Illinois River, Arkansas). Colonization of meiofauna was found to be rapid and generally increased with flow rates examined (1–2, 6–7, and 11–12 cm s⁻¹). Six of the 10 most abundant taxa successfully completed colonization in artificial channels (equaled or exceeded reference benthic densities) within 5 days. Benthic meiofauna were more abundant in fast flows in artificial channels and in fast and slow flows in reference stream sites. A diverse assemblage of meiofauna was collected from the plankton which was dominated by rotifers, copepods (mostly nauplii), dipterans, and cladocerans. Densities of drifting meiofauna (potential colonists of the benthos) were low (5 no. l⁻¹) and similar among artificial channels and reference sites regardless of flow rates (F _1,18 = 2.19, p = 0.1407). Although densities were low, the numbers of drifting meiofauna were more than sufficient to colonize the benthos. Less than 0.65% of the drifting meiofauna were needed to colonize the substrates of artificial streams. The benthic assemblage paralleled that of the plankton, consisting mainly of rotifers, copepods (mainly nauplii), and dipterans. Evidence for active control over dispersal was observed as meiofauna densities varied between the plankton and benthos over the diel cycle (F _1,18 = 6.02, p = 0.0001 and F _1,18 = 9.88, p = 0.006, respectively). Rotifers, copepods, and nematodes were more abundant in the plankton during the day and in the substrates at night. These results suggest that meiofauna assemblages can change rapidly in response to alterations of habitat patches by disturbance.     

Effect of domestic sewage on sand beach meiofauna at Goa,India

Meiofauna of a sewage-polluted sandy beach, where sand alone constituted > 90%, was surveyed. Nematodes dominated the fauna numerically at all stations, followed by harpacticoid copepods. Most of the animals were confined to the top 5 cm of the sediment. A seasonal pattern was observed in the distribution of the fauna. There were significant spatial and temporal variations in mean meiofauna density, attributed to organic discharge via sewage and prevailing environmental conditions in the study area.     

Long‐term moderate nutrient inputs enhance autotrophy in a forested Mediterranean stream

1. The effects of long‐term nutrient addition at moderate levels were examined in the food web of a forested Mediterranean stream. Basal concentrations of N and P were increased twofold (to c. 750 μg N) and threefold (to c. 30 μg P) from ambient concentrations in an experimental reach. Variations in the abundance of microbes (bacteria and algae), meiofauna and macrofauna, microbial processing of organic matter (extracellular enzyme activities) and stoichiometry of biofilms and invertebrates were compared to an upstream control reach during 4 years of artificial nutrient enhancement. 2. Effects were faster in the bacterial compartment but more substantial in the algal compartment. Epilithic algal biomass doubled in the enriched section jointly triggered by nutrients and increased light irradiance in winter and early spring. Only a few animal groups reacted to the enrichment, including the meiofaunal Copepoda, linked to their high use of enriched FPOM, and macrofaunal grazers (Ancylus), which followed the large algal biomass increase. 3. The enrichment caused biofilm phosphatase activity to decrease, while activities related to the use of algal‐related materials (peptidase, β‐glucosidase) increased. Enzymatic activities related to the use of allochthonous organic matter showed only minor and episodic increases. 4. Changes in stoichiometric ratios were apparent in the epilithic compartment, but not in the sand sediment or in the FPOM. Increases in P content were delayed for 9 months in epilithic biofilms and for nearly 2 years in the case of N. 5. After 2 years of enrichment, the flatworm Schmidtea polychroa (predator), oligochaetes (detritivore) and tadpoles of Bufo bufo (grazer) showed higher per cent N. 6. Enrichment effects were produced in spite of flow cessations that occurred commonly in summer. The results show that forested streams subjected to sustained (though minor) nutrient enrichment changed aspects of their biological structure and metabolism and that changes were especially favoured by periods when light was not limiting.     

Food of an endangered cyprinodont (Aphanius iberus): ontogenetic diet shift and prey electivity

Synopsis We studied the ontogenetic diet shift and prey electivity of an endangered cyprinodontid fish endemic to the Iberian Peninsula, the Spanish toothcarp (Aphanius iberus). The toothcarp’s diet was omnivorous, dominated by harpacticoid copepods (Mesochra lilljeborgi and Tisbe longicornis), copepod nauplii and detritus. Diet composition varied greatly among habitats, depending on prey availability. In a rarely inundated habitat (glasswort), there was more consumption of the isopod Protracheoniscus occidentalis and the harpacticoid copepod Mesochra lilljeborgi, while in algal mats another harpacticoid (Tisbe longicornis), chironomid dipterans and invertebrate eggs were more important in diet. Although a benthic feeding habitat has previously been suggested, in our study the diet was based rather on water column organisms for both glasswort and algal mat habitats. There was also an ontogenetic diet shift, with an increase of mean prey length with fish length, clearly linked to a microhabitat change. Smaller fish showed positive electivity and greater reliance on planktonic prey (e.g. copepod nauplii, the harpacticoid copepods Mesochra lilljeborgi and Tisbe longicornis, the rotifer Brachionus plicatilis, and ostracods), while larger fish elected and preyed on more benthic organisms (e.g. Canuella perplexa, Mesochra rapiens, and ephydrid dipterans).     

Meiofauna affect the macrobenthic biodiversity–ecosystem functioning relationship

The biodiversity–ecosystem functioning (B–EF) relationship has become of main interest in the last few decades, mostly because of the worldwide increase in species extinctions. However, most of these studies only consider species within single size‐class or trophic level, thereby most likely underestimating ecosystem complexity. To reach more realistic scenarios, we assessed the role of meiofauna (lower size‐class level) on the relationship between macrofaunal biodiversity and multiple benthic ecosystem properties. Experiments took place under controlled conditions using three macrofaunal species (Alitta (formerly Nereis) virens, Macoma balthica and Mya arenaria). A total of eight combinations of zero to three functionally different macrofaunal species were maintained in microcosms for 34 days in either the presence or absence of a different size‐class grouping (a meiofaunal mixture). The organic matter content and bacterial abundance in sediments and the oxygen and nutrient (NH₄⁺, NO_x^?, PO₄^3?) fluxes across the sediment‐water interface were measured and used as proxies of ecosystem properties. Overall, macrofaunal species richness did not modify any of the measured properties; however, we observed changes associated to the presence of A. virens and M. balthica. This study also revealed a strong impact of the presence of meiofauna on ecosystem properties. They changed interactions between macrofaunal species, which led to modifications in the ecosystem properties. Thus, even if this size‐class group has been poorly considered in previous studies, this experiment suggests that future studies should consider the meiofauna with greater attention, particularly in the context of B–EF.     

Experimental evidence of predation by juvenile flounder, Platichthys flesus, on a shallow water meiobenthic community

In the northern Baltic Sea juvenile flounder, Platichthys flesus, occur in high abundances on shallow sandy bottoms in late summer and autumn. They feed mainly on meiofauna and the ontogenetic switch to macrofaunal sized food occurs at a larger size here than in other areas, exemplifying the high relative importance of meiofauna. Consequently, juvenile P. flesus in the Baltic feed for a longer period on meiofauna, and could thus be expected to have a stronger predation effect on the meiofaunal assemblages. In this study the predation effects of juvenile P. flesus on meiofaunal abundances and community structure were studied using microcosms that were sampled repeatedly over a 3-week period. Significant differences between treatment and control were found for the total number of taxa, for abundances of harpacticoids, copepod nauplii and ostracods. The nematode community was not affected, but one genus, Axonolaimus, was negatively affected by predation. The predation affected meiofaunal community structure as the major taxon diversity was significantly reduced. The results suggest that the meiofauna on shallow sandy bottoms may be structured by juvenile P. flesus, and the combined predation pressure of juvenile flounder and other epibenthic predators in the area might be considerable. Microcosms are effective in testing natural predation, especially on meiofaunal communities, but field experiments should be conducted to account for the physical characteristics of the area studied.     

Meiofauna of springs in Finland in relation to environmental factors

The meiofauna was studied from springs in Finland which wascompletely ice-covered during the last glacial epoch. Of the31springs that were examined, 15 were in areas considered to bein anatural state and 16 had catchment areas subject toanthropogenicdisturbance from one of four sources: agriculture,inhabitation,gravel extraction or winter road de-icing with NaCl. The taxarecorded were a mixture of those already described fromepigeicwaters or groundwaters and some which may be regarded ascharacteristic of springs Statistical analysis revealeddifferencesin the meiofaunal taxa in relation to the water quality orcatchment area variables. Of the major meiofaunal groups thenematodes favoured springs influenced by agriculture,oligochaeteswere most tolerant of road de-icing, cladocerans and copepodspreferred springs in a natural state, ostracods couldwithstand theinfluence of road de-icing and gravel extraction, whilstdipteranlarvae could withstand the influence of agriculture.Oligochaetescomprised 13.4% of meiofaunal numbers. The most abundantspecieswas Aeolosoma quaternarium, which was most numerous insprings in a natural state, while Dero digitatapreferredeutrophic conditions. Harpacticoids comprised 14.1% ofindividualsand clearly preferred a natural state, Bryocamptusechinatusand Moraria brevipes being the most abundant species.Ostracods, of which the most abundant species was Potamocyprispallida, were the most numerous animal group, constituting21% ofthe meiofaunal population, and their numbers were greatest insprings influenced by road de-icing or gravel extraction.Dipteranlarvae, of which the majority were chironomids, comprised14.9% ofindividuals. The ostracods Potamocypris pallida andthreespecies of Candona and the harpacticoid Bryocamptuscuspidatus are new records for Finland.     

Ostracod reactions to non-toxic and toxic algae

Summary When fed algae continuously, laboratory ostracods had a life span of 21–28 days; without food, they were able to survive about 7 days. When exposed to thick suspensions of gas-vacuolate blue-green algae, survival rates were generally low. Twenty five % of our euplanktonic strains killed entire ostracod populations within 24–48 hrs; about 30% of other waterbloom strains were dangerous to ostracods causing either death or coordination loss. Some amphibious non-gas-vacuolated strains were also toxic to ostracods. Toxins from broken cells were not persistent in effect; ostracods surviving the initial shock often appeared recovered within 2–4 days. In di-algal systems, ostracods generally fed on a single species and did not always choose green algae over blue-green. Unicellular green algal strains (Ankistrodesmus, Scenedesmus, Kirchneriella) were generally preferred. Some chlorophycean forms (Zygnema, Hormidium, Trentepholia) were shunned in favor of bluegreens such as Tolypothrix and Westiella. Erect and branched strains (Fischerella, Westiella, Tolypothrix) were eaten before Anabaena and Nostoc with the latter genus being least preferred. Intact Nostoc colonies were usually not ingested, but some loosely growing, non-toxic strains of both Nostoc and Anabaena provided adequate food for ostracod populations indefinitely.