Cyanobacterial life at low O(2): community genomics and function reveal metabolic versatility and extremely low diversity in a Great Lakes sinkhole mat

Cyanobacteria are renowned as the mediators of Earth's oxygenation. However, little is known about the cyanobacterial communities that flourished under the low-O(2) conditions that characterized most of their evolutionary history. Microbial mats in the submerged Middle Island Sinkhole of Lake Huron provide opportunities to investigate cyanobacteria under such persistent low-O(2) conditions. Here, venting groundwater rich in sulfate and low in O(2) supports a unique benthic ecosystem of purple-colored cyanobacterial mats. Beneath the mat is a layer of carbonate that is enriched in calcite and to a lesser extent dolomite. In situ benthic metabolism chambers revealed that the mats are net sinks for O(2), suggesting primary production mechanisms other than oxygenic photosynthesis. Indeed, (14)C-bicarbonate uptake studies of autotrophic production show variable contributions from oxygenic and anoxygenic photosynthesis and chemosynthesis, presumably because of supply of sulfide. These results suggest the presence of either facultatively anoxygenic cyanobacteria or a mix of oxygenic/anoxygenic types of cyanobacteria. Shotgun metagenomic sequencing revealed a remarkably low-diversity mat community dominated by just one genotype most closely related to the cyanobacterium Phormidium autumnale, for which an essentially complete genome was reconstructed. Also recovered were partial genomes from a second genotype of Phormidium and several Oscillatoria. Despite the taxonomic simplicity, diverse cyanobacterial genes putatively involved in sulfur oxidation were identified, suggesting a diversity of sulfide physiologies. The dominant Phormidium genome reflects versatile metabolism and physiology that is specialized for a communal lifestyle under fluctuating redox conditions and light availability. Overall, this study provides genomic and physiologic insights into low-O(2) cyanobacterial mat ecosystems that played crucial geobiological roles over long stretches of Earth history.     

Macroalgal blooms alter community structure and primary productivity in marine ecosystems

Eutrophication, coupled with loss of herbivory due to habitat degradation and overharvesting, has increased the frequency and severity of macroalgal blooms worldwide. Macroalgal blooms interfere with human activities in coastal areas, and sometimes necessitate costly algal removal programmes. They also have many detrimental effects on marine and estuarine ecosystems, including induction of hypoxia, release of toxic hydrogen sulphide into the sediments and atmosphere, and the loss of ecologically and economically important species. However, macroalgal blooms can also increase habitat complexity, provide organisms with food and shelter, and reduce other problems associated with eutrophication. These contrasting effects make their overall ecological impacts unclear. We conducted a systematic review and meta‐analysis to estimate the overall effects of macroalgal blooms on several key measures of ecosystem structure and functioning in marine ecosystems. We also evaluated some of the ecological and methodological factors that might explain the highly variable effects observed in different studies. Averaged across all studies, macroalgal blooms had negative effects on the abundance and species richness of marine organisms, but blooms by different algal taxa had different consequences, ranging from strong negative to strong positive effects. Blooms' effects on species richness also depended on the habitat where they occurred, with the strongest negative effects seen in sandy or muddy subtidal habitats and in the rocky intertidal. Invertebrate communities also appeared to be particularly sensitive to blooms, suffering reductions in their abundance, species richness, and diversity. The total net primary productivity, gross primary productivity, and respiration of benthic ecosystems were higher during macroalgal blooms, but blooms had negative effects on the productivity and respiration of other organisms. These results suggest that, in addition to their direct social and economic costs, macroalgal blooms have ecological effects that may alter their capacity to deliver important ecosystem services.     

Influence of the Thin Drift Algal Mats on the Distribution of Macrozoobenthos in Kõiguste Bay, NE Baltic Sea

Macrozoobenthic communities within and outside of the drift algal mats were compared in Kõiguste Bay, NE Baltic Sea. The patches of the drift algae were on average 0.5–1 km wide in diameter covering about 25% of the total bottom area of the bay. Thickness of the mat did not exceed 6 cm. The biomass of the mat varied between 35 and 1391 g dw m^?2. The drift algal mats had no clear negative effect on macrozoobenthos except for a few infaunal species. The drift algae favoured several detrivorous, herbivorous and carnivorous species. Among the studied variables, the thickness of algal mat and oxygen concentration at near-bottom layer explained the best the structure of macrozoobenthos. Total number of invertebrate species increased curvilinearly with the thickness of algal mat having the peak value at 3–5 cm thick algal mat. To conclude, moderate drift algal mats increased habitat complexity and, thus, the diversity of benthic faunal assemblages in otherwise poorly vegetated coastal areas.     

Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole

For a large part of earth's history, cyanobacterial mats thrived in low‐oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sediment–water interface in modern ecosystems. The structure and function of benthic mats are shaped by biogeochemical processes in underlying sediments. A modern cyanobacterial mat system in a submerged sinkhole of Lake Huron (LH) provides a unique opportunity to explore such sediment–mat interactions. In the Middle Island Sinkhole (MIS), seeping groundwater establishes a low‐oxygen, sulfidic environment in which a microbial mat dominated by Phormidium and Planktothrix that is capable of both anoxygenic and oxygenic photosynthesis, as well as chemosynthesis, thrives. We explored the coupled microbial community composition and biogeochemical functioning of organic‐rich, sulfidic sediments underlying the surface mat. Microbial communities were diverse and vertically stratified to 12 cm sediment depth. In contrast to previous studies, which used low‐throughput or shotgun metagenomic approaches, our high‐throughput 16S rRNA gene sequencing approach revealed extensive diversity. This diversity was present within microbial groups, including putative sulfate‐reducing taxa of Deltaproteobacteria, some of which exhibited differential abundance patterns in the mats and with depth in the underlying sediments. The biological and geochemical conditions in the MIS were distinctly different from those in typical LH sediments of comparable depth. We found evidence for active cycling of sulfur, methane, and nutrients leading to high concentrations of sulfide, ammonium, and phosphorus in sediments underlying cyanobacterial mats. Indicators of nutrient availability were significantly related to MIS microbial community composition, while LH communities were also shaped by indicators of subsurface groundwater influence. These results show that interactions between the mats and sediments are crucial for sustaining this hot spot of biological diversity and biogeochemical cycling.     

The impact of macroalgal blooms on the use of the intertidal area and feeding behaviour of waders (Charadrii) in the Mondego estuary (west Portugal)

The present paper examined the possible impact of macroalgal blooms, among other influential factors, on prey abundance and availability to waders, assessing the consequences for feeding behaviour and for the specific patterns of use of the intertidal areas by these birds when macroalgal mats become dense and contiguous covering large areas of the Mondego estuary (west Portugal). Three representative microhabitats were chosen in intertidal flats to control and evaluate the effects of various factors on waders. This study shows that foraging waders did not seem to be indifferent to the effects of some biological factors, sediment characteristics and epistructures when selecting feeding microhabitats. Overall, the results suggest that the predominant and constant negative effects on specific patterns of use of the intertidal sub-areas by the majority of waders were determined by two main factors: gull perturbation and macroalgal biomass. The macroalgal blooms assumed, by their persistence over a period of several months, a different magnitude of effect when compared with sporadic factors such as gull presence. Contrary to the effects on distribution, macroalgal blooms do not influence negatively the feeding behaviour parameters considered. Moreover, we were unable to prove that the presence of macroalgae on the sediment was pernicious to the wader prey organisms, perhaps because the total amount of the area of the estuary affected by weed during the study period never exceeded 36 % and was surrounded by important algal-free areas that ensured a healthy prey population. Nevertheless, on the long-term, an increase of dense and contiguous macroalgal mats due to the progressive eutrophication of the Mondego estuary, covering large areas of the intertidal flats, may affect directly or indirectly all wader species.     

Effects of macroalgal mats on intertidal sandflats: an experimental study

The growth of green macroalgal mats is becoming increasingly common in many marine intertidal habitats. While the ecological effects of such growth has previously been experimentally investigated on mudflats, such experiments have rarely been performed on intertidal sandflats. This study investigated the ecological effects of macroalgal cover on a moderately exposed intertidal sandflat, Drum Sands, Firth of Forth, Scotland. Artificially implanted Enteromorpha prolifera (Müller) caused marked changes in the macrobenthos, together with significant changes in all the measured sediment variables. After 6 weeks, the weed significantly increased the macrofaunal diversity. The numbers of Pygospio elegans (Claparède) were significantly reduced under weed mats, while those of Capitella capitata (Fabricius), oligochaetes and gammarids increased. Percent water, organics and silt/clay contents, medium phi and sorting coefficient significantly increased in the sediments under weed mats which also became significantly more reduced between 1 and 8 cm depth. After 20 weeks, a macrofaunal community numerically dominated by C. capitata, with a significantly reduced diversity, was present under weed mats, while sediment variables were no longer significantly different from controls. The negative effect of E. prolifera on P. elegans was mainly due to larval filtering, suggesting that weed is likely to have detrimental effects on population maintenance of most species which rely on planktonic larval recruitment. These results are broadly similar to those obtained from algal manipulation experiments performed in much more sheltered, muddier environments. We suggest that a predictable deterioration in environmental quality results from the growth of macroalgal mats in soft-bottom habitats. However, the longer term effects of such algal growth are less predictable and depend upon the spatial distributions of the most abundant infaunal species and the spatial heterogeneity of weed mat establishment.     

Production and Consumption of Dimethylsulfoniopropionate in Marine Microbial Mats

The fate of dimethylsulfoniopropionate (DMSP), a major sulfonium compound in marine ecosystems, was examined in Microcoleus chthonoplastes-dominated microbial mats. Chemical decomposition of DMSP was observed under laboratory conditions at pH values higher than 10.0. pH profiles measured in situ showed that these highly alkaline conditions occurred in microbial mats. Axenic cultures of M. chthonoplastes contained 37.3 μmol of DMSP g of protein⁻¹, which was partially liberated when the cells were subjected to an osmotic shock. DMSP-amended mat slurries showed a rapid turnover of this compound. The addition of glutaraldehyde blocked DMSP decrease, indicating biological consumption. Populations of potential dimethyl sulfide consumers were found in the top 10 mm of the mat.     

Dynamics of Dimethyl Sulfide in a Marine Microbial Mat

Abstract The microbial mat was chosen as a model ecosystem to study dynamics of dimethyl sulfide (DMS) in marine sediments in order to gain insight into key processes and factors which determine emission rates. A practical advantage, compared to open ocean ecosystems, is that microbial mats contain high biomasses of different functional groups of bacteria involved in DMS dynamics, and that DMS concentrations are generally high enough to allow direct measurement of emission rates. Field data showed that, during the seasonal development of microbial mats, concentrations of chlorophyll a corresponded to dimethylsulfoniopropionate (DMSP). DMSP is an important precursor of DMS. It was demonstrated, with laboratory cultures, that various species of benthic diatoms produce substantial amounts of DMSP. The abundances of aerobic and anaerobic DMS- or DMSO-utilizing bacteria were estimated using the most-probable-number technique. Laboratory experiments with relatively undisturbed sediment cores showed that microbial mats act as a sink for DMS under oxic/light (day) conditions, and as a source of DMS under anoxic/dark (night) conditions. Axenic culture studies with Chromatium vinosum M2 and Thiocapsa pfennigii M8 (isolated from a microbial mat) showed that, under anoxic/light conditions, DMS was quantitatively converted to dimethylsulfoxide (DMSO). T. roseopersicina M11 converted DMSP to DMS and acrylate, apparently without use of either substrate. Received: 5 May 1997; Accepted: 21 August 1997     

Low-molecular-weight sulfonates, a major substrate for sulfate reducers in marine microbial mats.

Several low-molecular-weight sulfonates were added to microbial mat slurries to investigate their effects on sulfate reduction. Instantaneous production of sulfide occurred after taurine and cysteate were added to all of the microbial mats tested. The rates of production in the presence of taurine and cysteate were 35 and 24 microM HS(-) h(-1) in a stromatolite mat, 38 and 36 microM HS(-) h(-1) in a salt pond mat, and 27 and 18 microM HS(-) h(-1) in a salt marsh mat, respectively. The traditionally used substrates lactate and acetate stimulated the rate of sulfide production 3 to 10 times more than taurine and cysteate stimulated the rate of sulfide production in all mats, but when ethanol, glycolate, and glutamate were added to stromatolite mat slurries, the resulting increases were similar to the increases observed with taurine and cysteate. Isethionate, sulfosuccinate, and sulfobenzoate were tested only with the stromatolite mat slurry, and these compounds had much smaller effects on sulfide production. Addition of molybdate resulted in a greater inhibitory effect on acetate and lactate utilization than on sulfonate use, suggesting that different metabolic pathways were involved. In all of the mats tested taurine and cysteate were present in the pore water at nanomolar to micromolar concentrations. An enrichment culture from the stromatolite mat was obtained on cysteate in a medium lacking sulfate and incubated anaerobically. The rate of cysteate consumption by this enrichment culture was 1.6 pmol cell(-1) h(-1). Compared to the results of slurry studies, this rate suggests that organisms with properties similar to the properties of this enrichment culture are a major constituent of the sulfidogenic population. In addition, taurine was consumed at some of highest dilutions obtained from most-probable-number enrichment cultures obtained from stromatolite samples. Based on our comparison of the sulfide production rates found in various mats, low-molecular-weight sulfonates are important sources of C and S in these ecosystems.     

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.     

Disturbance and recovery of microbial community structure and function following Hurricane Frances

Disturbance and recovery influence microbial community structure and ecosystem functions in most natural environments. This study from a hypersaline Bahamian lagoon details the response of a benthic cyanobacterial mat to disturbance by Hurricane Frances, a category-4 storm. Clone libraries of cyanobacterial small subunit r-RNA genes and nitrogenase genes revealed significant shifts in cyanobacterial and diazotroph community composition following the hurricane. Post-hurricane clone libraries were dominated by sequences that had been rare in pre-hurricane communities. In spite of this dominance shift, re-colonizing mat communities performed nitrogen fixation and photosynthesis at rates within the normal range of variation measured in the mat at similar salinities. There was a tendency for nitrogen fixation rates from mats re-colonizing sites with hurricane-related sand deposition to be higher than those from mats re-colonizing sites without significant sand deposition. This suggests that the altered communities responded to a carbon : nitrogen imbalance that was particularly pronounced in areas subjected to disturbance by sand burial. The post-hurricane dominance of organisms that had been previously rare suggests that pre-hurricane diversity and functional redundancy contributed to the rapid recovery of ecosystem function in the post-disturbance environment.     

Effects of the invasive duckweed Lemna minuta on aquatic animals: evidence from an indoor experiment

Abstract

Since effects of alien invasive free-floating plants can be relevant in aquatic ecosystems, we investigated the non-native Lemna minuta impact on four aquatic animal groups: Hydra vulgaris (Coelenterates), Asellus aquaticus (Arthropods), Gambusia affinis (Fish), Bufo bufo tadpoles (Amphibians). An indoor experiment was conducted keeping animals in water held in tanks with L. minuta mats of 0.5 (WI1), 1.5?cm thick (WI2) and without mats (WOU). Water parameters (DO, DO%, pH) and animal responses (survival rate, vitality) were measured every 48?h (0–288?h). Treatments with mats showed significant impacts on animals which were more severe with increasing mat thickness. Strong decreasing of oxygen and pH associated with mat occurrence had a large impact on animals. In WI2 all individuals died within 144?h (H. vulgaris, B. bufo within 96h), while in WI1 there was a higher survival rate and vitality (excluding B. bufo died within 96?h) and in WOU no deaths. This evidence suggests L. minuta thick mats (≥1.5?cm) could have a high impact on animal biodiversity, especially reducing oxygenation level in aquatic ecosystem.     

Effects of green algal mats on bivalves in a New England mud flat

Concurrent with the spread of green algal mats on tidal flats, reports of macrofauna dieoffs under dense algal mats have increased in numbers. Bivalves seem to be particularly affected by persistent dense algal mats. Bivalve species with a long extendible siphon seem to be less affected underneath algal mats, but no distinction has been made in the past between species with short and those with long siphons,Mya arenaria andMacoma balthica, on an intertidal mudflat in New England. Abundances ofM. arenaria declined substantially during the study period when a thick green algal mat covered the mudflat for several months. Numbers of the small bivalveGemma gemma also decreased substantially, whereas abundances ofM. balthica showed minimal variation during the time of algal coverage. In algae removal/addition experiments numbers ofM. arenaria decreased, but effects were only significant in an algal addition to previously algal-free mudflat areas. Abundance ofM. balthica did not change significantly in the algal removal/additition experiments. Over the time period of the experiment (9 weeks),M. arenaria showed measurable size increase in uncovered mudflat areas, but not underneath algal mats. Similarly,M. balthica only increased in size in the uncovered mudflat area. From these results it is concluded thatM. balthica can survive time periods of dense algal coverage because it is able to penetrate through the algal mat with its long extendible siphon, and thus can reach well-oxygenated water layers above the mat.M. arenaria with its thick, less extendible, siphon cannot push through dense algal mats and therefore is more likely to die underneath persistent algal mats.     

Biogeochemistry and community composition of iron- and sulfur-precipitating microbial mats at the Chefren mud volcano (Nile Deep Sea Fan, Eastern Mediterranean)

In this study we determined the composition and biogeochemistry of novel, brightly colored, white and orange microbial mats at the surface of a brine seep at the outer rim of the Chefren mud volcano. These mats were interspersed with one another, but their underlying sediment biogeochemistries differed considerably. Microscopy revealed that the white mats were granules composed of elemental S filaments, similar to those produced by the sulfide-oxidizing epsilonproteobacterium "Candidatus Arcobacter sulfidicus." Fluorescence in situ hybridization indicated that microorganisms targeted by a "Ca. Arcobacter sulfidicus"-specific oligonucleotide probe constituted up to 24% of the total the cells within these mats. Several 16S rRNA gene sequences from organisms closely related to "Ca. Arcobacter sulfidicus" were identified. In contrast, the orange mat consisted mostly of bright orange flakes composed of empty Fe(III) (hydr)oxide-coated microbial sheaths, similar to those produced by the neutrophilic Fe(II)-oxidizing betaproteobacterium Leptothrix ochracea. None of the 16S rRNA gene sequences obtained from these samples were closely related to sequences of known neutrophilic aerobic Fe(II)-oxidizing bacteria. The sediments below both types of mats showed relatively high sulfate reduction rates (300 nmol x cm(-3) x day(-1)) partially fueled by the anaerobic oxidation of methane (10 to 20 nmol x cm(-3) x day(-1)). Free sulfide produced below the white mat was depleted by sulfide oxidation within the mat itself. Below the orange mat free Fe(II) reached the surface layer and was depleted in part by microbial Fe(II) oxidation. Both mats and the sediments underneath them hosted very diverse microbial communities and contained mineral precipitates, most likely due to differences in fluid flow patterns.     

Sedimentary pyrite sulfur isotope compositions preserve signatures of the surface microbial mat environment in sediments underlying low-oxygen cyanobacterial mats

The sedimentary pyrite sulfur isotope (δ³⁴S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite δ³⁴S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide δ³⁴S geochemistry. Pyrite δ³⁴S values often capture δ³⁴S signatures of porewater sulfide at the location of pyrite formation. However, microbial mats are dynamic environments in which biogeochemical cycling shifts vertically on diurnal cycles. Therefore, there is a need to study how the location of pyrite formation impacts pyrite δ³⁴S patterns in these dynamic systems. Here, we present diurnal porewater sulfide δ³⁴S trends and δ³⁴S values of pyrite and iron monosulfides from Middle Island Sinkhole, Lake Huron. The sediment–water interface of this sinkhole hosts a low-oxygen cyanobacterial mat ecosystem, which serves as a useful location to explore preservation of sedimentary pyrite δ³⁴S signatures in early Earth environments. Porewater sulfide δ³⁴S values vary by up to ~25‰ throughout the day due to light-driven changes in surface microbial community activity that propagate downwards, affecting porewater geochemistry as deep as 7.5 cm in the sediment. Progressive consumption of the sulfate reservoir drives δ³⁴S variability, instead of variations in average cell-specific sulfate reduction rates and/or sulfide oxidation at different depths in the sediment. The δ³⁴S values of pyrite are similar to porewater sulfide δ³⁴S values near the mat surface. We suggest that oxidative sulfur cycling and other microbial activity promote pyrite formation in and immediately adjacent to the microbial mat and that iron geochemistry limits further pyrite formation with depth in the sediment. These results imply that primary δ³⁴S signatures of pyrite deposited in organic-rich, iron-poor microbial mat environments capture information about microbial sulfur cycling and environmental conditions at the mat surface and are only minimally affected by deeper sedimentary processes during early diagenesis.     

Disparate distributions of chemolithotrophs containing form IA or IC large subunit genes for ribulose-1,5-bisphosphate carboxylase/oxygenase in intertidal marine and littoral lake sediments

The distributions of bacterial form IA and form IC ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) were investigated using Lowes Cove intertidal mudflat and Damariscotta Lake littoral sediments by PCR amplification of 492-495 bp fragments of the large subunit RuBisCO gene, cbbL. Genomic extracts for amplification were obtained from lake surface (upper 2 mm), mudflat surface (upper 2 mm), subsurface (5-7 cm), and soft-shell clam (Mya arenaria) burrow-wall sediments, as well as from a sulfide-oxidizing mat. Phylogenetic analyses of cbbL clone libraries revealed that Lowes Cove sediments were dominated by form IA cbbL-containing sequences most closely related to cbbL genes of sulfur-oxidizing bacteria or sulfide-oxidizing mats. In contrast, Damariscotta Lake cbbL clones contained primarily form IC cbbL sequences, which typify aerobic CO- and hydrogen-oxidizing facultative chemolithotrophs. Statistical analyses supported clear differentiation of intertidal and lake chemolithotroph communities, and provided evidence for some differentiation among intertidal communities. amova and libshuff analyses of Lowes Cove libraries suggested that M. arenaria burrow-wall sediments did not harbour distinct communities compared with surface and subsurface sediments, but that surface and subsurface libraries displayed moderate differences. The results collectively support a conceptual model in which the relative distribution of form IA- and IC-containing bacterial chemolithotrophs depends on sulfide availability, which could reflect the role of sulfate reduction in sediment organic matter metabolism, or the presence of geothermal sulfide sources.     

Benthic and pelagic food resources for zooplankton in shallow high-latitude lakes and ponds

1. Shallow lakes and ponds are a major component of the northern landscape and often contain a high zooplankton biomass despite clear waters that are poor in phytoplankton. 2. In this study we quantified zooplankton food sources and feeding rates in the shallow waters of two contrasting high‐latitude biomes: subarctic forest tundra (Kuujjuarapik, Quebec) and high arctic polar desert (Resolute, Nunavut). Five substrate types were tested (beads, bacteria, picophytoplankton, filamentous plankton and microbial mats). Special attention was given to the role of benthos, a component that is usually poorly integrated into models of aquatic foodwebs. 3. Consistent with observations elsewhere in the circumpolar region, high concentrations of adult macrozooplankton occurred in all sites (up to 17 100 crustaceans m^?3) while phytoplankton concentrations and primary productivity were low. The communities were composed of multiple species, including Daphnia middendorfiana, Hesperodiaptomus arcticus, Leptodiaptomus minutus, Artemiopsis stefanssoni and Branchinecta paludosa. 4. Detritus made 89–98% of the planktonic resource pool and bacteria contributed the highest biomass (up to 29 mg C m^?3) of the planktonic food particles available to zooplankton. Benthic resources were dominated by microbial mats that grew in nutrient‐rich conditions at the base of the ponds and which dominated overall ecosystem biomass and productivity. 5. All species were flexible in their feeding but there were large, order of magnitude differences in clearance rates among taxa. These differences likely resulted from different grazing strategies among cladocerans, copepods and fairy shrimps, and possibly also from adaptation to specific food types and size ranges that occur locally in these waters. 6. The subarctic cladocerans Ceriodaphnia quadrangula and D. middendorfiana, and the arctic fairy shrimp B. paludosa were observed to graze directly on the microbial mats and the feeding experiments confirmed their assimilation of benthic substrates. The other zooplankton species showed a more pelagic feeding mode but were capable of using microbial mat filaments, thus may be indirectly linked to benthic processes via resuspension. 7. Our study indicates that the classical aquatic food web in which phytoplankton provide the sole production base for grazers does not apply to northern shallow lakes and ponds. Instead, microbial mats increase the physical complexity of these high latitude ecosystems and likely play a role in sustaining their high zooplankton biomass.     

Subsidy hypothesis and strength of trophic cascades across ecosystems

Ecosystems are differentially open to subsidies of energy, material and organisms. This fundamental ecosystem attribute has long been recognized but the influence of this property on community regulation has not been investigated. We propose that this environmental attribute may explain variation in the strength of trophic cascades among ecosystems. Simply because of gravity, we should predict that systems with convex profiles receive low amounts of subsidies whereas systems with concave profiles act as spatial attractors, and receive high amounts of subsidies. The subsidy hypothesis states that ecosystems with high amounts of allochthonous inputs will experience the strongest trophic cascades. To test this hypothesis, we derive ecosystem models and investigate the effect of location and magnitude of subsidies on the strength of trophic cascades. Predictions from our models support the subsidy hypothesis and highlight the need to consider ecosystems as open to allochthonous flows.