Understorey benthic microalgae and their consumers depend on habitat complexity and light in a microtidal coastal ecosystem

Presence of habitat-forming macroalgae is supposed to mitigate effects of altered resources on benthic microalgae and their consumers. In a field experiment in a microtidal area of the Western Baltic Sea, we tested the interactive effects of nutrient enrichment, artificial shading, and habitat complexity on microalgal biomass and diversity as well as invertebrate abundance and richness. Habitat complexity comprised three levels, the presence of macroalgal canopy of Fucus vesiculosus, the presence of macroalgal propagules, and the absence of both (=control). Microalgal biomass (and richness) was significantly reduced by canopy presence (−88%, compared to control) and shading (−42%), with the highest biomass in the absence of both canopy and macroalgal propagules at ambient light. Within the microalgal assemblage, higher biomass was related to lower evenness (higher dominance). Density of two main invertebrate groups (snails and amphipods) strongly increased with canopy presence (on average from 53 to 154 individuals m⁻² stone area for snails, and from 234 to 1203 individuals m⁻² for amphipods) and so did invertebrate richness (from 4.3 to 10.3). Additionally, snail density doubled with increasing light availability. The snail responses to light and canopy were independent, the former relating to higher availability of microalgal prey, the latter to more structure. Microalgal taxon richness and biomass decreased with increasing invertebrate richness and with density of snails and amphipods. Our experiment thus showed that the presence of habitat-forming macroalgae alters biomass and diversity across trophic levels in benthic coastal communities as well as their response to resource manipulations.     

Macroalgal detrital systems: an overlooked ecological niche for heterotrophic nitrogen fixation

Diazotrophic macroalgal associations (DMAs) can contribute fixed nitrogen (N) to the host macroalgae. Biological nitrogen fixation (BNF) rates investigated using acetylene reduction assays with living macroalgae surrounding Santa Catalina Island were low (maximum: 36 nmol N × g⁻¹ (dw) × h⁻¹) and probably insufficient towards helping meet macroalgal N demand. However, DMAs were observed during periods of low nitrate availability in Southern California coastal waters, highlighting the potential importance of diazotrophs during N depleted conditions. Eleven long-term (16–32 days) litter bag decomposition experiments with various macroalgae, especially those with high (> 10) C:N ratios, resulted in much higher BNF rates (maximum: 693 nmol N × g⁻¹ (dw) × h⁻¹) than observed with living macroalgae. BNF rates were lower at the beginning of macroalgal decomposition but rapidly increased during the second phase before declining towards the end of decomposition. Labile carbon availability is likely influencing BNF rates throughout macroalgal degradation and limits BNF in the final decomposition stage. Comparable dark and light BNF rates with most macroalgae surveyed suggest macroalgal detrital systems are an overlooked, potentially global, niche for heterotrophic N₂ fixation. Lastly, suppressed BNF rates with sodium molybdate additions highlight the prevalence of sulfate reducing diazotrophs.     

THE EFFECT OF A HORN SNAIL ON ULVA EXPANSA (CHLOROPHYTA): CONSUMER OR FACILITATOR OF GROWTH?1

To determine whether California horn snails are more likely to be consumers or facilitators of Ulva expansa (Setch) S. & G. growth in estuaries, we conducted manipulative experiments that evaluated algal growth and the movement of N between the water column, algal tissue, and, in the second experiment, sediments. Algae grew poorly in the absence of sediments, drawing on their own sequestered N supplies (3.5% of dry weight reduced to <2%) and N released by snails and by depleting inorganic N in the water column. There was no evidence of consumption when snail densities ranged from 0 to 900.m^?2 (0, 3, 6, and 9 per aquarium), as algal growth was similar for all snail densities, and snail lengths did not increase during the 21–d experiment. when sediment was provided, N was depleted in the sediment and enhanced in the algal tissue. As in the first experiment, the water column was depleted of inorganic N and enriched with organic N, mostly in the dissolved form. Because both snails and macroalgae often dominate the shallow waters of southern California's lagoons and estuaries, our evidence that the snails are primarily facilitators of algal growth (via transfer of N from sediments to the water column) suggests that snails may play an important role in both food web and N dynamics.     

The Seasonal Dynamics and Trophic Relations of the Plankton Components in Lake Peipsi (Peipus)

The seasonal dynamics of the biomass and production of phyto-, zoo- and bacterioplankton was investigated during the vegetation periods (from May to November) in 1985 and 1986 in the pelagial of the large eutrophic lake Peipsi (Estonia). The average values of productions per vegetation period for the investigation years were as follows: phytoplanktion − 203.5 gC · m⁻²; bacterioplankton − 37.9 gC · m⁻²; filter-feeding zooplankton − 20.6 gC · m⁻² and predatory zooplankton − 1.5 gC · m⁻². The herbivorous zooplankton production constituted 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain — filtrators are feeding mostly on living algae and the detrital food chain seems of little importance. The dominance of large forms (Melosira sp., Aphanothece saxicola), in the phytoplankton during the major part of the vegetation period is assumed to be a result of high grazing pressure on small algae. Zooplankton grazing was investigated in situ in a specially constructed twin bathometer. Experimental measurements revealed, that zooplanktion presence in the experimental vessel actually stimulated the phytoplankton growth in many cases — the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), excreted by the concentrated zooplankton in the grazing chamber, which led to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11%. Grazing on bacteria increased, when grazing on phytoplankton was somehow disturbed.     

UPTAKE AND RELEASE OF NITROGEN BY THE MACROALGAE GRACILARIA VERMICULOPHYLLA (RHODOPHYTA)1

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using ¹⁵N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non‐native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back‐barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μ mol N·g dw^?1·d^?1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μ mol N·g dw^?1·d^?1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m^?2·d^?1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours).     

The relative importance of nutrient enrichment and herbivory on macroalgal communities near Norman's Pond Cay, Exumas Cays, Bahamas: a “natural” enrichment experiment

The simultaneous effects of grazing and nutrient enrichment on macroalgal communities were experimentally investigated using plastic mesh enclosure/exclosure cages along a natural nutrient (DIN, SRP) gradient from the discharge of a tidal mangrove creek on the west side of Norman's Pond Cay, Exumas Cays, Bahamas. Nutrient enrichment was the only factor affecting total biomass of macroalgae whereas selective herbivory moderated species composition. Biomass ranged from >2 kg dry weight m⁻² at the DIN-enriched Waterfall to <600 g dry weight m⁻² in the DIN-limited Algal Halo. Grazing by the queen conch Strombus gigas and the sea hare Aplysia dactylomela significantly reduced biomass of the epiphyte Laurencia intricata, but not its host Digenea simplex at the DIN-enriched Waterfall. These two rhodophytes dominated the macroalgal community at the DIN-enriched Waterfall and inner Algal Halo, whereas the chlorophytes Microdictyon marinum and Cladophora catenata dominated the relatively DIN-depleted outer Algal Halo. Higher grazing rates by S. gigas, A. dactylomela, and juvenile parrotfish and surgeonfish at the DIN-enriched Waterfall compared to the more oligotrophic Algal Halo suggested selective grazing on higher quality (nitrogen) diets by these herbivores. A pronounced increase in the carbon/nitrogen (C:N) ratios of D. simplex and L. intricata from the Waterfall to the Algal Halo indicated increased DIN-limitation of growth in the more offshore, lower DIN macroalgal communities. In contrast, only modest increases in C:N ratios of M. marinum and C. catenata between the Waterfall and Algal Halo suggested that these species are better adapted to growth in reef environments with lower levels of DIN enrichment. Low coral cover (<2%) co-occurred with high macroalgal cover (>35%) at DIN-enriched reef sites around Norman's Pond Cay (Waterfall, Patch Reef, North Perry Reef) compared to relatively high coral cover (10-20%) and low macroalgal cover (<20%) at the lower DIN reef sites (Rainbow Reef, Lang's Reef). These results support ecological theory that bottom-up control via nutrient enrichment is a primary factor regulating overall biomass and taxonomic assemblages of macroalgae on coral reefs, whereas grazing is more important in controlling relative species composition via dietary preferences.     

Effect of grazing by isopods and amphipods on growth of Ulva spp. (Chlorophyta)

Eutrophication of shallow coastal waters often leads to blooms of macroalgae. Grazing by crustaceans, such as amphipods and isopods, can reduce macroalgal biomass accumulation. At the same time, growth of the macroalgae can be stimulated by epiphyte removal. The role of grazing by isopods and amphipods on Ulva spp. biomass development was investigated in the Veerse Meer, a brackish lagoon situated in the southwest Netherlands. Exclusion of grazing in the field did not stimulate Ulva spp. growth. In fact, growth rates were higher in exclosures that allowed grazers to enter. Edibility tests identified the amphipod Gammarus locusta, and the isopods Idotea chelipes and Sphaeroma hookeri as potential grazers on Ulva spp. However, when epiphytic diatoms were present on the Ulva spp. thalli, Gammarus and Sphaeroma grazed on ephiphytes and not on Ulva tissue. Only Idotea continued to graze on Ulva spp. A laboratory growth experiment revealed a positive effect of Gammarus presence on Ulva spp. growth, probably caused by preferential removal of epiphytic diatoms from the Ulva spp. thalli. The growth stimulation by epiphyte removing grazers such as Gammarus may explain the higher growth rates in the presence of grazers observed in the field. When determining the potential role of invertebrate grazers in controlling macroalgal biomass accumulation, it is important to include an assessment of the epiphyte abundance on the macroalgae, as preferential removal of epiphytes may stimulate growth and thus have the opposite effect.     

Oxygen and ammonium dynamics during a farming cycle of the bivalve <Emphasis Type="Italic">Tapes philippinarum</Emphasis>

Fluxes of dissolved oxygen and ammonium across the water sediment interface were measured in a control and in an experimental area farmed with the clam Tapes philippinarum. Young clams were seeded in March 2003 at mean (~500 ind m⁻²) and high (~1500 ind m⁻²) densities in a sandy area (2100 m²) of the Sacca di Goro Lagoon, Italy. Approximately every two months, until October 2003, intact sediment cores were collected and incubated in the light and in the dark and surface sediments (0–2 cm) were analysed for organic matter and nitrogen content. Clams farming induced pronounced changes in sediment characteristics and metabolism. Oxygen consumption and ammonium production at the high density area were, on average, 3 to 4 and 1.9 to 4.9 folds higher than those measured in the control field respectively; rates were positively correlated with clams biomass. Experimental fields resulted “Net and Total Heterotrophyc” in 3 out of 4 sampling dates and clams were the major factor shifting the benthic system towards this status. In only one occasion the appearance of the macroalgae Ulva spp. pushed the system rapidly towards hyperautotrophic conditions. Our results indicated that clams have the potential to drive benthic metabolism in farmed areas and to sustain macroalgal growth through regeneration of a limiting nutrient for seawater as inorganic N.     

Macroalgal-mediated transfers of water column nitrogen to intertidal sediments and salt marsh plants

In many temperate estuaries, mats of opportunistic macroalgae accumulate on intertidal flats and in lower elevations of salt marshes, perhaps playing a role in linking water column nitrogen (N) supply to these benthic habitats. Using a flow-through seawater system and tidal simulator, we varied densities (equivalent to 0, 1, 2, or 3 kg m⁻² wet mass) of ¹⁵N-labelled macroalgae (Enteromorpha intestinalis) on estuarine sediments in microcosms with/without pickleweed (Salicornia virginica) to assess N transfers from algae. In the 6-week experiment, macroalgal biomass increased from initial levels in the lower density treatments but all algae lost N mass, probably through both leakage and decomposition. With all densities of algae added, sediments and pickleweed became enriched in ¹⁵N. With increasing mat density, losses of algal N mass increased, resulting in stepwise increases in ¹⁵N labeling of the deeper sediments and pickleweed. While we did not detect a growth response in pickleweed with macroalgal addition during the experiment, N losses from algal mats that persist over many months and/or recur each year could be important to the mineral nutrition of N-limited marsh plants. We conclude that N dynamics of intertidal sediments and lower salt marsh vegetation are linked to the N pools of co-occurring macroalgae and that further study is needed to assess the magnitude and importance of N transfers.     

Impact of invasive apple snails in Hong Kong on wetland macrophytes,nutrients, phytoplankton and filamentous algae

1. Grazing by invasive species can affect many aspects of an aquatic system, but most studies have focused on the direct effects on plants. We conducted mesocosm and laboratory experiments to examine the impact of the invasive apple snail Pomacea canaliculata on macrophytes, filamentous algae, nutrients and phytoplankton. 2. In a freshwater pond, we confined 500 g of Myriophyllum aquaticum or Eichhornia crassipes with 0, 2, 4 or 8 apple snails in 1 m × 1 m × 1 m enclosures for approximately 1 month. Apple snails grazed heavily on both species of macrophytes, with higher overall weight losses at higher snail densities. The damage patterns differed between the two macrophytes. In M. aquaticum, both leaves and stems suffered from substantial herbivory, whereas in E. crassipes, only the roots suffered significant weight reduction. 3. In addition to grazing on macrophytes, apple snails appeared to have controlled the growth of filamentous algae, as these did not develop in the snail treatments. The ability of P. canaliculata to control filamentous algae was supported by a laboratory experiment where the consumption was as high as 0.25 g g⁻¹ snail DW d⁻¹. Because of a lack of native herbivorous snails in the pond, the growth of filamentous algae (mainly Spirogyra sp.) reached 80.3 g m⁻², forming a spongy pond scum in the no‐apple snail control. Together with previous reports that apple snails could eat the juveniles and eggs of other freshwater snails, our results indicated that P. canaliculata could have out‐competed native herbivorous snails from the pond by predation on their juveniles or eggs. Alternatively, P. canaliculata might have out‐competed them by monopolisation of food resources. 4. Nitrogen and phosphorous concentrations remained low throughout both experiments and were not correlated with apple snail density. The treatment effects on chlorophyll a (Chl a) and phytoplankton composition varied in the two experiments. In the M. aquaticum experiment, with increasing snail density, Chl a increased, and the phytoplankton community became dominated by Cryptophyceae. In the E. crassipes experiment, Chl a level was independent of snail density, but with increasing snail density, the phytoplankton community became co‐dominated by Cryptophyceae, Chlorophyceae and Bacillariophyceae. 5. Given the multiple effects of P. canaliculata on wetland biodiversity and function, management strategies should be developed to prevent its further spread. In invaded wetlands, strategies should be developed to eradicate the apple snail and re‐introduce native snails which can control the development of filamentous algae.     

THE PERFORMANCE OF SPRAY‐IRRIGATED ULVA LACTUCA (ULVOPHYCEAE,CHLOROPHYTA) AS A CROP AND AS A BIOFILTER OF FISHPOND EFFLUENTS1

The seaweed Ulva lactuca L. was spray cultured by mariculture effluents in a mattress‐like layer, held in air on slanted boards by plastic netting. Air‐agitated seaweed suspension tanks were the reference. Growth rate, yield, and ammonia‐N removal rate were 11.8% · d^?1, 171 g fresh weight (fwt) · m^?2 · d^?1, and 5 g N · m^?2 · d^?1, respectively, by the spray‐cultured U. lactuca, and 16.9% · d^?1, 283 g fwt · m^?2 · d^?1, and 7 g N · m^?2 · d^?1, respectively, by the tank U. lactuca. Biomass protein content was similar in both treatments. Dissolved oxygen in the fishpond effluent water was raised by >3 mg · L^?1 and pH by up to half a unit, upon passage through both culture systems. The data suggest that spray‐irrigation culture of U. lactuca in this simple green‐mattress‐like system supplies the seaweed all it needs to grow and biofilter at rates close to those in standard air‐agitated tank culture.     

Short Term Bioturbation Activity in the Lagoonal Sediments of Tikehau Atoll (Tuamotu Archipelago,French Polynesia)

To quantify bioturbation activity in Tikehau lagoon, a tracer made of black basaltic sand was poured over the natural white calcareous sediment surface. Three stations respectively located on the inner flat (-3m), the inner slope (-9m), and the lagoon floor (-19m), were studied for short periods of time (48 hours). Bioturbation by macrofauna was quantified by volume of sediment ejected onto the experimental surface and by volume of tracer incorporated into sediment. The results showed a rapid incorporation of sedimented particles at the interface by way of the funnels and burrows of surface deposit feeders and carnivores. Expelled quantities varied with respect to site location: 213 cm³ · m⁻² · 24 h⁻¹ in the inner flat; 98.9 cm³ · m⁻² · 24 h⁻¹ in the inner slope; 7.9 m³ · m⁻² · 24 h⁻¹ in the lagoon floor. Bioturbation by decapod megafauna appeared to be important in the dynamics of the sediments in the deepest areas of the lagoon. In these areas, with almost no hydrodynamical impacts on sediments, bioturbating events were responsible for sediment mixing (despite lower absolute rates than in shallow area). Hydrodynamics controlled the spatial distribution of macroinvertebrate trophic groups by its effects on sedimentation.     

Selective grazing by the mud snail Ilyanassa obsoleta

Summary Mud snails (Ilyanassa obsoleta) starved for 48 h were allowed to feed on sediments in laboratory microcosms. Sediment cores sliced at 2 mm intervals were compared to snail stomach contents for per cent carbon and nitrogen, plant pigment contents and species composition of benthic diatoms. Concentrations of carbon, nitrogen, phaeopigments, phycocyanin and chlorophyll were enriched in the top 2 mm of the sediments compared to 7–10 mm depth by a factor of 2–10. In turn, these materials were 20–40 times more concentrated in snail guts than in the surface sediments. Snail feces were enriched for carbon and nitrogen by 5–7 times over the surface sediments. Bacterial chlorophyll peaked at about 3–4 mm in the sediments and was not detectable in the snail stomach contents. The C/N ratio of the snail stomach contents was only 6 compared to a ratio of 8.5 for their feces and 12 for the surface sediments.The percentage of migratory diatoms (e.g. Nitzschia and Navicula) decreased with depth where non-migratory species, such as Fragilaria pinnata, dominated. These migratory species were more common in the snails than in the sediments on which they were feeding.A comparison of daily ingestion rates to the animal's energy budget shows that this selective ingestion is sufficient to meet Ilyanassa's energy needs.     

High Nitrogen and Phosphorous Acquisition by Belowground Parts of Caulerpa prolifera (Chlorophyta) Contribute to the Species' Rapid Spread in Ria Formosa Lagoon,Southern Portugal

Despite worldwide proliferation of the genus Caulerpa and subsequent effects on benthic communities, little is known about the nutritional physiology of the Caulerpales. Here, we investigated the uptake rates of ammonium, nitrate, amino acids, and phosphate through the fronds and rhizoids + stolon, the internal translocation of nitrogen, and developed a nitrogen budget for the rapidly spreading Caulerpa prolifera in Ria Formosa lagoon, southern Portugal. Caulerpa prolifera acquired nutrients by both aboveground and belowground parts at similar rates, except nitrate, for which fronds showed 2-fold higher uptake rates. Ammonium was the preferential nitrogen source (81% of the total nitrogen acquisition), and amino acids, which accounted for a significant fraction of total N acquisition (19%), were taken up at faster rates than nitrate. Basipetal translocation of ¹⁵N incorporated as ammonium was nearly 3-fold higher than acropetal translocation, whereas ¹⁵N translocation as nitrate and amino acids was smaller but equal in either direction. The estimated total nitrogen acquisition by C. prolifera was 689 μmol · m⁻² · h⁻¹, whereas the total nitrogen requirement for growth was 672 μmol · m⁻² · h⁻¹. The uptake of ammonium and amino acids by belowground parts accounted for the larger fraction of the total nitrogen acquisition of C. prolifera and is sufficient to satisfy the species nitrogen requirements for growth. This may be one reason explaining the fast spreading of the seaweed in the bare sediments of Ria Formosa where it does not have any macrophyte competitors and the concentration of nutrients is high.     

The influence of periphyton biomass and density on grazing in Physella virgata

We studied how differences in periphyton colonization interval and snail density affected grazing rates in Physella virgata, and whether snails controlled periphyton biomass. Both egestion rates and incorporation rates of ¹⁴C labeled periphyton were estimated in laboratory experiments. Periphyton biomass increased with field colonization interval in all experiments, but did not consistently influence estimates of grazing rate. However, increased periphyton abundance in one of the experiments could still explain higher grazer rates in that year, although larger snail body size is a confounding explanation. Increased snail density also resulted in decreased grazing rates, as observed in earlier studies with this snail species, as well as in studies with other snail grazers. Our results suggest grazing rates and resulting impacts may change seasonally with variation in either periphyton biomass, grazer life-history stage or population density.     

The effect of horse, cattle and sheep grazing on the diversity and abundance of land snails in nutrient-poor calcareous grasslands

Livestock grazing is a common management practise in semi-natural grasslands in Central Europe. Different types of livestock (horses, cattle, sheep) and grazing intensity are known to affect the richness and composition of plant species. However, knowledge of grazing-dependent effects on invertebrates is limited. We examined the influence of horse, cattle and sheep grazing on the richness, abundance and composition of land snail species in 21 calcareous nutrient-poor grassland areas in the northwestern Jura Mountains, Switzerland. Grazing by different livestock species did not affect the species richness, abundance and species composition of land snails. Furthermore, the number of open-land species and the ratio of large- to small-sized snail species or individuals did not differ among the three pasture types. However, independent of livestock species, grazing intensity negatively influenced the snail fauna. Snail species richness, abundance and number of Red list species decreased with increasing grazing intensity. Grazing intensity also affected the occurrence of individual snail species (Truncatellina cylindrica, Cecilioides acicula, Candidula unifasciata and Trichia plebeia). To preserve the snail fauna in nutrient-poor grasslands, pastures can be stocked with horses, cattle or sheep. However, both maximum stocking rate (number of livestock units per hectare) and grazing duration (number of grazing days per year) must be carefully defined for the proper management of the pastures.     

The effect of natural populations of the burrowing and grazing soldier crab (Mictyris longicarpus) on sediment irrigation, benthic metabolism and nitrogen fluxes

The aim of this study was to determine the effect of sediment grazing and burrowing activities of natural populations of Mictyris longicarpus on benthic metabolism, nitrogen flux and irrigation rates by comparing sediments taken from minimum disturbance exclusion cages and adjacent sediments subject to M. longicarpus activities. M. longicarpus reduced sediment surface chlorophyll a (approximately 77%), organic carbon (approximately 95%) and total nitrogen concentrations (approximately 99%) in comparison to ungrazed sediments. Consequently, they significantly reduced gross benthic O₂ production (about 71%) and sediment O₂ consumption (approximately 46%). Mean N₂ fluxes showed net effluxes (276-430 μmol m⁻² day⁻¹) in the presences of M. longicarpus and net uptakes (194.09-449.21 μmol m⁻² day⁻¹) where they were excluded. The net uptake of N₂ was most likely due to cyanobacteria fixing of N₂, as dense microbial mats became established over the sediment surface in the absence of M. longicarpus grazing activity. Sediment irrigation/transport rates calculated from CsCl tracer dilution indicated greater irrigation rates in the exclusions (12.12-16.22 l m⁻² h⁻¹) compared to inhabited sediments (6.33-11.73 l m⁻² h⁻¹) and this was again was most likely due to the lack of grazing pressure which allowed large populations of small burrowing polychaetes to inhabit the organic matter rich exclusion sediments. As such, the main influence of M. longicarpus was the interception and consumption of transported organic material, benthic microalgae and other small infaunal organisms resulting in the removal of approximately 0.06 g m⁻² day⁻¹ of nitrogen and 12.12 g m⁻² day⁻¹ of organic carbon. This “cleansing” of the sediments reduced sediment metabolism and the flux of solutes across the sediment water interface and ultimately the heavy predation of M. longicarpus by transient species such as stingrays, results in a net loss of carbon and nitrogen from the system.     

Annual cycles of benthic community oxygen uptake in a deep oligotrophic lake (Attersee,Austria)

Benthic community oxygen uptake of Lake Attersee sediments was measured between 1976 and 1979, along two profiles at 25, 50 and 100 m depth. Profile I was situated in the bay of Unterach into which the main tributary, Mondsee-Ache, discharges a high load of organic matter. Profile II was chosen at Weyregg to avoid the eutrophying effect of Mondsee-Ache. Oxygen uptake rates of Unterach sediments at 25 and 50 m depth were found to be higher when compared to the other sites (mean rates: Unterach 25 m = 15.56, 50 m = 11.05 mg O₂ · m⁻² · h⁻¹; Weyregg 25 m = 6.43, 50 m = 5.14 mg O₂ · m⁻² · h⁻¹). Organic content of the uppermost sediment layer was also higher in the bay of Unterach than at Weyregg. Oxygen uptake rates of undisturbed sediment cores vary considerably throughout the year, but no simple correlation existed with variations in organic content of the sediments. Peaks of organic matter were found to concur with following peaks of oxygen uptake rates, which implies that a certain time span is necessary for transforming freshly sedimented organic matter into a state digestable for the benthic community. The retardation between increasing organic matter of the sediment and the corresponding increase of benthic oxygen uptake was different at Unterach and Weyregg respectively, which is explained by the different quality of sedimenting material.     

Indirect effects of a parasite on a benthic community: an experiment with trematodes, snails and periphyton

1. Few studies have directly addressed the role played by parasites in the structure and function of ecosystems. Parasites influence the behaviour, reproduction and overall fitness of their hosts, but have been usually overlooked in community and ecosystem‐level studies. We investigated the effects of trematode parasites on snail–periphyton interactions. 2. Physa  acuta (Gastropoda: Pulmonata) snails infected with the trematode Posthodiplostomum minimum (often >30% of within‐shell biomass) grazed more rapidly than uninfected snails. Trematode effects on snail grazing indirectly affected the standing stock and community structure of periphyton. Populations of snails with 50% infected individuals reduced algal biomass by 20% more than populations with lesser (10% or 0%) infection rates. 3. The alga Cladophora glomerata dominated periphyton communities grazed by snail populations with 50% infection rates, whereas diatoms and blue–green algal taxa dominated when grazed by snail populations with lower infection rates. 4. Thus, trematodes indirectly affected periphyton communities by altering host snail behaviour, a trait‐mediated indirect effect. These results indicate that trematodes can indirectly influence benthic community structure beyond simple population fitness, with possible related effects on ecosystem function.     

Cd transfer in the deposit-feeder Prosobranch Hydrobia ulvae (Pennant) from benthic diatoms: the kinetics of rapid Cd assimilation and efflux

The kinetics of Cd trophic transfer from benthic diatoms to the Prosobranch mud snail Hydrobia ulvae was described experimentally in microcosms using Cd contaminated microalgae (0.71, 3.63 and 8.54 μg Cd mg Chl a⁻¹). The depurated mud snails (2 ind. cm⁻²) were allowed to feed on the stable Cd pre-contaminated benthic diatoms at the concentration of 2 mg Chl a dm⁻³ to ensure that the algal food availability was not a limiting factor. Weight-specific ingestion rate (IR) and assimilation efficiency (AE) were calculated by an indirect mass-balance method on the basis of metal residues in the snail tissues, and metal loss (efflux rate, Δe) was estimated for the time intervals when a decrease or no change in the tissue metal concentrations occurred.A similar pattern of consumption was observed in all experiments: ingestion was rapid over the first 4 h, followed by slower ingestion period (between 4 and 16 h). The feeding behaviour of H. ulvae was not affected by the different diatom Cd concentrations. An analogous two-phasic pattern was observed in the tissue Cd concentration changes. Net accumulation of Cd in the snails was observed for the two highest exposures, indicating that the Cd threshold concentration in food above which metal is retained in the body, lies between 0.71 and 3.63 μg Cd mg Chl a⁻¹. The respective 16-h AEs were 0.024% and 0.004% potentially due to rapid gut-passage of microalgae and/or diminished nutritional value of the food. The efflux rates, calculated for the last 12 h of exposure, were positively related to the concentration of Cd in the snail tissues and microalgae. This study demonstrated that trophic transfer should be considered as a source of Cd accumulation in snails and the ability of H. ulvae to enhance their rate of Cd elimination in response to elevated metal concentrations in the ambient environment is relevant for models predicting metal bioaccumulation and toxicity in coastal and estuarine systems.