Subarctic Ponds Research Articles

Assessing long-term diatom changes in sub-Arctic ponds receiving high fluxes of seabird nutrients.

Algal bioindicators, such as diatoms, often show subdued responses to eutrophication in Arctic lakes because climate-related changes (e.g., ice cover) tend to be the overriding factors influencing assemblage composition. Here, we examined how sub-Arctic ponds historically receiving high nutrient inputs from nesting seabirds have responded to recent climate change. We present diatom data obtained from 12 sediment cores in seaduck-affected ponds located on islands through Hudson Strait, Canada. All study cores show consistently elevated values of sedimentary ẟ15N, an established proxy for tracking marine-derived nutrients, indicating seabirds have been present on these islands for at least the duration of the sediment records (~100 to 400 years). We document diverse epiphytic diatom assemblages to the base of all sediment cores, which is in marked contrast to seabird-free Arctic ponds-these oligotrophic sites typically record epilithic diatom flora prior to recent warming. Diatoms are likely responding indirectly to seabird nutrients via habitat as nutrients promote the growth of mosses supporting epiphytic diatom communities. This masks the typical diatom response to increased warming in the Arctic, which also results in habitat changes and the growth of mosses around the pond edges. Changes in sedimentary chlorophyll a were not consistently synchronous with large changes in ẟ15N values, suggesting that primary production in ponds is not responding linearly to changes in seabird-derived nitrogen. Across all ponds, we recorded shifts in diatom epiphytic assemblages (e.g., increases in % relative abundance of many Nitzschia species) that often align with increases in chlorophyll a. The changes in diatoms and chlorophyll a, although variable, are most likely driven by climate change as they are generally consistent with longer ice-free conditions and growing seasons. Together, our results show that to effectively use diatoms in animal population reconstructions across the sub-Arctic and Arctic, a strong understanding of eutrophication and climate change, based on supplementary proxies, is also required.

The effect of habitat on benthic diatoms in subarctic ponds – differences between rock and sediment

Subarctic ecosystems represent a vulnerable environment for their biota and studying their habitats provides a better understanding of ongoing and accelerated environmental change. Literature suggests that habitat and environmental variables such as water chemistry jointly influence benthic algal species richness and assemblage composition. However, the combined effects of habitat and water chemistry on benthic algal communities are poorly understood especially in subarctic freshwaters. This study compared benthic diatom assemblages on rocks and on sediment and examined if environmental variables affect diatoms similarly in these habitats. A total of 20 subarctic ponds were sampled for diatoms on sediment and rock in summer 2022 in northern Fennoscandia. Differences in species richness and Pielou evenness between habitats were analysed by a paired Wilcoxon signed-rank test. The most significant environmental variables influencing diatom species richness were determined by generalized linear models. Differences in diatom assemblage compositions were analysed using non-metric multidimensional scaling, analysis of similarities, and Jaccard similarity index. Finally, to study how environmental variables shape diatom compositions on rock and sediment, a redundancy analysis was performed. According to our results species richness and evenness were significantly higher on sediment than on rock, and water pH was the major determinant for diatom richness in both habitats. Diatom assemblage composition did not differ between rocks and sediment, and composition was shaped by the same environmental variables such as pond surface area and water pH in both habitats. Our results support the prevailing conception that sediment habitat contains higher diatom species richness than rocks, while assemblage composition does not differ between these habitats. This study highlights the role of water pH on benthic diatoms regardless of their habitat. Given that global warming is expected to affect water pH levels, such a change will probably have a notable influence also on subarctic diatom communities in the future decades.

Patterns and drivers for benthic algal biomass in sub-Arctic mountain ponds

This study investigated the spatial variation in total benthic algal biomass and within cyanobacteria, green algae, and diatoms in sub-Arctic ponds. Additionally to more widely used explanatory variables, snowmelt and ice duration were considered as their importance on algal communities is poorly understood. The data comprised algal biomasses from 45 sub-Arctic ponds in the Finnish Lapland. A generalized linear model and hierarchical partitioning were used to identify the significantly influential variables. Cyanobacteria were the most abundant algal group. Trace elements (e.g. Fe, Al, and Mn) were the most significant explanatory variable group in explaining algal biomasses. Macronutrients apart from K were found insignificant in all models. There were positive relationships between some algal biomasses indicating no strong competition between them. Snow and ice variables were found insignificant for all models, but they could have an important secondary role on algal communities. The results highlight the importance of trace elements in shaping algal biomasses in sub-Arctic ponds and thus their wider use in research can be advocated to better understand the productivity of nutrient poor and acidic waters in sub-Arctic regions. Focussing on benthic algal biomasses and the chemical composition of sub-Arctic freshwaters provides important information on the aquatic primary production.

Elevational microbial β diversity and community assembly processes in subarctic ponds

Abstract Knowledge about elevational β diversity patterns and processes is limited for microorganisms, especially in sensitive aquatic ecosystems at high latitudes. Our aim is to investigate elevational β diversity patterns and underlying assembly processes for three biofilm microbial groups in subarctic freshwater ponds. We sampled 89 ponds covering an elevational gradient from 10 to 1038 m above sea level in subarctic Finland and Norway. The microbial groups studied included benthic diatoms, cyanobacteria, and bacteria (i.e., bacteria other than cyanobacteria). We expected to find elevational distance decay in community similarity, driven primarily by variation in the local environment. We further tested whether the level of β diversity decreased towards higher elevations, and whether the relative influence of deterministic and stochastic processes on community assembly changed along the elevational gradient, with increasing effect of deterministic processes at higher elevations under more extreme environmental conditions. As statistical methods, we used generalised dissimilarity modelling, Mantel tests, and a null model with Raup–Crick dissimilarities. For all microbial groups, community dissimilarities increased with increasing environmental or elevational distances. The pure effects of environment were significant for all groups, and pH was the most important variable affecting community dissimilarities. Mean community dissimilarities generally decreased with elevation for all microbial groups. The communities were influenced by both deterministic and stochastic processes, the former of which was stronger towards higher elevations. The influence of deterministic processes was strongest for bacteria. Generally, the communities under deterministic control were more similar to each other than expected by chance. Elevational β diversity patterns of subarctic pond microorganisms primarily reflected environmental effects, showing decreasing β diversity towards high elevations owing to stronger environmental filtering. The relative effects of deterministic or stochastic assembly processes varied among microbial groups, highlighting the need for including multiple groups when studying biodiversity patterns and underlying ecological processes.

Observing diatom diversity and community composition along environmental gradients in subarctic mountain ponds

Abstract Cold subarctic pond ecosystems will be threatened due to the increase in global temperatures. Therefore, it is important to gain more knowledge on how their biota may respond to global warming. The aim of this research was to illustrate the variability in diatom species richness and community composition along environmental gradients in northernmost Finland and Norway and reveal the variables most strongly associated with diatom biodiversity. We also compared diatom biodiversity among different biotope types and placed emphasis on the distribution of rare diatom taxa. A total of 100 subarctic ponds from the Finnish and Norwegian Lapland covering an elevational gradient of 8–887 m above sea level were sampled. A generalised linear model and hierarchical partitioning were used to identify variables associated with diatom species richness. To identify variables associated with diatom community composition, a non‐metric multidimensional scaling ordination was performed. Finally, a principal component analysis and permutational multivariate analysis of variance were used for the investigation of environmental and biotic differences among biotope types. Water pH, aluminium concentration, and air temperature best explained the variation in species richness and community composition. Diatom species richness and community composition did not differ among the biotope types, but environmental variables in pine forests differed significantly from other biotope types. Many diatom species occupied the entire elevational gradient, while rare taxa seemed to appear at the ends of the elevational gradient as well around the mid‐elevational zone. We found that elevation was not sufficient to explain the variation in diatom species richness and community composition, but diatom biodiversity was shaped by a variety of local‐scale environmental variables, some of which are in turn correlated with elevation. Our findings suggest that local abiotic factors and especially water chemistry are important factors in describing the variability in diatom community composition and species richness in subarctic ponds. It seems that the lowest and the highest elevations provide suitable habitats for rare diatom taxa that have unique environmental preferences but some of the rare taxa were also constrained to mid elevations. Our study provides new insights into the role of local abiotic variables in shaping subarctic mountain pond biodiversity. We urge researchers not only to study elevational gradients per se in mountain areas, but also pay special attention to environmental covariates that may play a notable role in maintaining freshwater biodiversity in the subarctic.

The Role of Methane Transport From the Active Layer in Sustaining Methane Emissions and Food Chains in Subarctic Ponds

AbstractGroundwater discharge from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH4) into Arctic lakes and streams, but its contribution to CH4 emissions from thaw ponds and its influence on the trophic support and nutritional quality of pond food chains remains unexplored. We quantified the transport of CH4 from the active layer through groundwater discharge into thaw ponds in a subarctic catchment in northern Sweden, using radon (222Rn) as groundwater tracer. We analyzed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of groundwater‐mediated CH4 inputs on the aquatic food chains. Our results indicate that active layer groundwater discharge flows are nontrivial (range 6%–46% of pond volume per day) and the associated CH4 fluxes (median 339 mg C m−2day−1, interquartile range [IQR]: 179–419 mg C m−2 day−1) can sustain the diffusive CH4 emissions from most of the ponds (155 mg C m−2 day−1, IQR: 55–234 mg C m−2 day−1). Consumers in ponds receiving greater CH4 inputs from the active layer had lower stable carbon (C) isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Overall, this work links physical (CH4 transport from the active layer), biogeochemical (CH4 emission), and ecological (MOB‐consumer interaction) processes to provide direct evidence for the role of active layer groundwater discharge in CH4 cycling of subarctic thaw ponds.

Permafrost thaw stimulates primary producers but has a moderate effect on primary consumers in subarctic ponds

AbstractFrozen tundra soils hold one of the Earth's largest pools of organic carbon. Climate warming and the associated permafrost thaw release a large fraction of this carbon into circumpolar lakes, inducing extreme browning that fuels the heterotrophic microbial food web. How this permafrost carbon affects organisms higher in the food chain remains unknown. Using dissolved organic matter properties, total phosphorus, chlorophyll a, fatty acids, and stable isotopes, we investigated the influence of thawing permafrost on primary producers and primary consumers of the planktonic food web. We sampled four subarctic thaw ponds that were affected by permafrost carbon and another four ponds that were not. Our results highlight the stimulating influence of eroding and degrading ice‐rich permafrost on nutrients and planktonic algae. Relative to the non‐thaw ponds, the permafrost thaw‐influenced freshwaters had higher total phosphorus concentrations (14.8 vs. 70.4 µg/L, respectively). This in turn led to a higher chlorophyll a (2.7 vs. 45.2 µg/L) and seston omega‐3 fatty acid concentrations (7.3 vs. 53.5 µg/L) despite significantly reduced light for primary production. Differences between the thaw and non‐thaw ponds were less marked at the primary consumer level. Daphnia pulex, which dominated the crustacean zooplankton community, did not respond to the higher omega‐3 availability in the thaw ponds but rather assimilated the high‐quality fatty acids equally in all ponds, possibly because their metabolic needs were already saturated. However, some lower quality terrestrial carbon compounds from permafrost ended up in the D. pulex body mass, resulting in a median allochthony of 18% based on fatty acid mixing model; non‐thaw ponds had median allochthony mixing model estimates of 8%. The high availability of algal resources seemed to prevent extensive zooplankton allochthony in subarctic thaw ponds.

Emergence of steeply stratified permafrost thaw ponds changes zooplankton ecology in subarctic freshwaters

ABSTRACT Climate change and associated permafrost thaw are creating new shallow waterbodies in vast regions of the circumpolar Arctic. These thaw ponds are characterized by high concentrations of colored dissolved organic matter originating from the degrading watershed, inducing a strong vertical thermal and oxygen (O2) stratification. We investigated the zooplankton community and biomass in eight subarctic thaw ponds and evaluated how zooplankton respond to this stratification. In a subset of three ponds, we further examined how other environmental variables, including essential fatty acids (EFA) concentration and phytoplankton, bacteria, and larval phantom midge Chaoborus biomass stratify and contribute to the vertical distribution of zooplankton in this increasingly common type of arctic freshwater system. The zooplankton community was extremely abundant in all ponds (up to 3,548 ind L−1) and dominated mainly by rotifers (35–93 percent of the biomass). Most zooplankton aggregated at the interface between the shallow well-oxygenated mixed surface layer and the deeper hypoxic but algal-rich stratified layer, and their distribution was affected by a combination of O2, Chaoborus, phytoplankton, and EFA that were supplied from opposite directions. Our findings show how water column stratification deeply affects the ecology of planktonic organisms in circumpolar freshwaters and indicate Arctic zooplankton species composition is expected to deeply change with the ongoing warming and browning.

Effects of shoreline permafrost thaw on nutrient dynamics and diatom ecology in a subarctic tundra pond

In permafrost landscapes, ponds and lakes are ubiquitous and important features that support long-term carbon storage and diverse diatom communities. With climate warming, thermokarst lake expansion may modify the ability of these aquatic systems to maintain these functions, as a consequence of changes to nutrient inputs and cycling. A recent (2006–2008) thermokarst shoreline expansion event adjacent to small Frisbee Pond, in the Hudson Bay Lowlands, Canada, presented an opportunity to evaluate the effects of thaw-driven disturbance on pond nutrient dynamics. A sediment core was collected in 2015 and analysed for diatom assemblage composition, total nutrient (carbon, nitrogen and phosphorus) accumulation rates, and stable isotopes of organic carbon (δ13C) and nitrogen (δ15N). Results indicate that recent shoreline expansion in this subarctic peatland pond caused an increase in total diatom accumulation, although shifts in diatom community composition were initiated prior to the shoreline slump, particularly an increase in the relative abundance of Denticula kuetzingii. Nutrient accumulation rates in the sediment increased dramatically both prior to and following shoreline slumping, with isotope values indicating an increased supply of allochthonous carbon and atmospheric nitrogen. Investigations of hydroclimate conditions prior to the shoreline slump indicate warming of summer air temperatures and an increase in precipitation over the long-term mean, consistent with changing climate conditions that caused thermokarst disturbance in other permafrost environments. This study has important implications for understanding hydro-ecological responses of small surface water bodies to general climate forcing, in combination with rapid thermokarst shoreline expansion, under a changing climate.

Elevational patterns and hierarchical determinants of biodiversity across microbial taxonomic scales.

Microbial biogeography is gaining increasing attention due to recent molecular methodological advance. However, the diversity patterns and their environmental determinants across taxonomic scales are still poorly studied. By sampling along an extensive elevational gradient in subarctic ponds of Finland and Norway, we examined the diversity patterns of aquatic bacteria and fungi from whole community to individual taxa across taxonomic coverage and taxonomic resolutions. We further quantified cross-phylum congruence in multiple biodiversity metrics and evaluated the relative importance of climate, catchment and local pond variables as the hierarchical drivers of biodiversity across taxonomic scales. Bacterial community showed significantly decreasing elevational patterns in species richness and evenness, and U-shaped patterns in local contribution to beta diversity (LCBD). Conversely, no significant species richness and evenness patterns were found for fungal community. Elevational patterns in species richness and LCBD, but not in evenness, were congruent across bacterial phyla. When narrowing down the taxonomic scope towards higher resolutions, bacterial diversity showed weaker and more complex elevational patterns. Taxonomic downscaling also indicated a notable change in the relative importance of biodiversity determinants with stronger local environmental filtering, but decreased importance of climatic variables. This suggested that niche conservatism of temperature preference was phylogenetically deeper than that of water chemistry variables. Our results provide novel perspectives for microbial biogeography and highlight the importance of taxonomic scale dependency and hierarchical drivers when modelling biodiversity and species distribution responses to future climatic scenarios.

Cell size and acid tolerance constrain pond diatom distributions in the subarctic

AbstractNiche characteristics and traits affect species distributions. However, their effects on the distribution of microorganisms remain understudied, especially in subarctic regions. Our aim was to determine the effects of species niche characteristics and traits on benthic diatom (Bacillariophyceae) elevational distributions, and to identify the key drivers of trait diversity across climate‐sensitive subarctic ponds.We sampled 102 ponds along an elevational gradient of 10 to 1,038 m a.s.l. in subarctic Finland and Norway. We calculated the niche characteristics by Outlying Mean Index analysis using an independent data set collected from 141 lakes in northern Fennoscandia. We ran generalised linear models to identify traits most strongly affecting the distribution of individual species, and used a functional dispersion metric to explore trait diversity at community level.Benthic diatom distributions correlated positively with niche breadth, implying that generalists were more widely distributed than specialists, but negatively with niche position, suggesting that marginal species had narrow distributions. Cell size and acid tolerance were the key traits affecting diatom distributions: small size and strong acid tolerance were associated with a wider distribution.Local abiotic variables were more important than elevation in shaping trait diversity, which increased withpHand decreased with conductivity. Several acidic wetland ponds located at low elevations exhibited functionally clustered communities, reflecting stressful conditions that comprised a filter that selected acid‐tolerant species.Our results suggest that benthic diatom distributions across subarctic ponds are jointly driven by species cell size and tolerance towards water chemistry variables, especially acidity. Although elevation summarises the effects of several environmental factors, local abiotic variables were more strongly associated with trait diversity than were elevation effects. Our findings provide new insights into the key mechanisms associated with aquatic microbial distributions in highly sensitive climatic regions.

Organic matter accumulation and salinity change in open water areas within a saline boreal fen in the Athabasca Oil Sands Region, Canada

Saline boreal fens represent potential models for post-mining landscape reclamation in the Athabasca Oil Sands Region (Canada) where wetland construction is challenged by salinization. One of the key indicators of reclamation success is the accumulation of organic matter within constructed fens. Thus, a better understanding of the linkages between salt content and organic matter storage in saline boreal fens can be useful for advancing fen construction in this region. As such, this study investigates how salinity fluctuations estimated by diatom-based transfer functions, coupled with changes in hydrological conditions and vegetation inferred from macrofossils have influenced organic matter accumulation rates (OMAR) over the last ~100 years in open-water areas (ponds) within a saline boreal fen near Fort McMurray, Alberta. Median OMAR (181 g m−2 yr−1) of the site suggests that the ponds situated within saline boreal fens can accumulate organic matter at a rate comparable to freshwater boreal and subarctic ponds, and the estimated salinity levels (3–21 ppt) did not severely affect organic matter accumulation. Strong significant positive (Lager Pond), strong significant negative (South Pond), and weak insignificant (Pilsner Pond) correlations between OMAR and diatom-inferred salinity were observed, suggesting that relations between organic matter accumulation and salt content are not straightforward, and salinity was not the main control on OMAR. Macrofossil data showed that organic matter accumulation has been mainly driven by water level, type of primary producers and pond regime. OMAR was the highest during the transition from peatland to ponds due to low decomposition rates resulting from high inputs of relatively resistant plant litter, and anoxic conditions. A macrophyte-dominated pond regime was associated with higher OMAR relative to algae-dominated regime.

Local and geographical factors jointly drive elevational patterns in three microbial groups across subarctic ponds

AbstractAimElevational biodiversity patterns are understudied in high‐latitude aquatic systems, even though these systems are important for detecting very early impacts of climatic changes on Earth. The aim of this study was to examine the elevational trends in species richness and local contribution to beta diversity (LCBD) of three biofilm microbial groups in freshwater ponds and to identify the key mechanisms underlying these patterns.LocationOne hundred and forty‐six ponds in subarctic Finland and Norway distributed across the tree line along an elevational gradient of 10–1,038 m a.s.l., spanning from forested landscape to barren boulder fields.Time periodJuly–August 2015.Major taxa studiedDiatoms, cyanobacteria and non‐cyanobacteria.MethodsGeneralized linear models were used to identify the most important pond variables explaining richness and LCBD. Structural equation models were used to explore the direct and indirect effects of multiscale drivers on richness and LCBD.ResultsDiatom and cyanobacteria richness showed unimodal elevational patterns, whereas non‐cyanobacteria richness decreased with increasing elevation. The LCBD–elevation relationship was U‐shaped for all three microbial groups. Diatom and cyanobacteria richness and LCBD were best explained by local pond variables, especially by pH. Non‐cyanobacteria richness and LCBD were related to pond variables, elevation as a proxy for climatic conditions, and normalized difference vegetation index as a proxy for terrestrial productivity.Main conclusionsAquatic autotrophs were primarily controlled by environmental filtering, whereas heterotrophic bacteria were also affected by terrestrial productivity and elevation. All studied aspects of microbial diversity were directly or indirectly linked to elevation; therefore, climatic changes may greatly alter aquatic microbial assemblages.

Unexpected differences in the population genetics of phasmavirids (Bunyavirales) from subarctic ponds.

Little is known of the evolution of RNA viruses in aquatic systems. Here, we assess the genetic connectivity of two bunyaviruses (Kigluaik phantom orthophasmavirus or KIGV and Nome phantom orthophasmavirus or NOMV) with zooplanktonic hosts from subarctic ponds. We expected weak genetic structure among populations as the hosts (phantom midges) have a terrestrial winged dispersal stage. To test whether their respective viruses mirror this structure, we collected and analyzed population datasets from 21 subarctic freshwater ponds and obtained sequences from all four genes in the viral genomes. Prevalence averaged 66 per cent for 514 host specimens and was not significantly different between recently formed thaw ponds and glacial ponds. Unexpectedly, KIGV from older ponds showed pronounced haplotype divergence with little evidence of genetic connectivity. However, KIGV populations from recent thaw ponds appeared to be represented by a closely related haplotype group, perhaps indicating a genotypic dispersal bias. Unlike KIGV, NOMV had modest structure and diversity in recently formed thaw ponds. For each virus, we found elevated genetic diversity relative to the host, but similar population structures to the host. Our results suggest that non-random processes such as virus–host interactions, genotypic bias, and habitat effects differ among polar aquatic RNA viruses.

Arctic and Sub-Arctic shallow lakes in a multiple-stressor world: a paleoecological perspective

Considerable progress has been made using experimental approaches and spatial surveys to better understand and manage shallow lake ecosystems. Meanwhile, research on the environmental histories of shallow lakes has lagged behind paleolimnological studies conducted on deeper systems, with most studies on shallow systems being completed over the last decade or so. Although shallow lakes may pose additional challenges to paleoenvironmental assessments, these potential shortcomings can often be identified and addressed. Due to their size, shallow lakes may act as amplifiers for a variety of environmental stressors, while dampening others. This overview summarizes a few recent studies assessing the long-term effects of multiple stressors on shallow lake ecosystems, with a particular focus on Arctic and Sub-Arctic ponds. Issues related to climatic change (e.g., the rapid transformation of shallow ponds in polar regions), as well as recent paleolimnological studies using shallow lake sediments to track past flooding frequencies, assessing the effects of seabirds and other biovectors on terrestrial ecosystems, blending archaeology and paleolimnology, and investigating the paths and trajectories of different pollutants (e.g., contaminants from the burgeoning petroleum industry) are highlighted. Some potential avenues for future research are also discussed.

High Methylmercury in Arctic and Subarctic Ponds is Related to Nutrient Levels in the Warming Eastern Canadian Arctic.

Permafrost thaw ponds are ubiquitous in the eastern Canadian Arctic, yet little information exists on their potential as sources of methylmercury (MeHg) to freshwaters. They are microbially active and conducive to methylation of inorganic mercury, and are also affected by Arctic warming. This multiyear study investigated thaw ponds in a discontinuous permafrost region in the Subarctic taiga (Kuujjuarapik-Whapmagoostui, QC) and a continuous permafrost region in the Arctic tundra (Bylot Island, NU). MeHg concentrations in thaw ponds were well above levels measured in most freshwater ecosystems in the Canadian Arctic (>0.1 ng L(-1)). On Bylot, ice-wedge trough ponds showed significantly higher MeHg (0.3-2.2 ng L(-1)) than polygonal ponds (0.1-0.3 ng L(-1)) or lakes (<0.1 ng L(-1)). High MeHg was measured in the bottom waters of Subarctic thaw ponds near Kuujjuarapik (0.1-3.1 ng L(-1)). High water MeHg concentrations in thaw ponds were strongly correlated with variables associated with high inputs of organic matter (DOC, a320, Fe), nutrients (TP, TN), and microbial activity (dissolved CO2 and CH4). Thawing permafrost due to Arctic warming will continue to release nutrients and organic carbon into these systems and increase ponding in some regions, likely stimulating higher water concentrations of MeHg. Greater hydrological connectivity from permafrost thawing may potentially increase transport of MeHg from thaw ponds to neighboring aquatic ecosystems.

Nutrient Uptake and Short-Term Responses of Phytoplankton and Benthic Algal Communities from a Subarctic Pond to Experimental Nutrient Enrichment in Microcosms

Climate warming is anticipated to affect high-latitude regions, including abundant ponds of the Hudson Bay Lowlands (HBL). However, it remains unclear if associated increased frequency of nutrient pulses will be rapidly consumed by aquatic biota and sediment or lead to a rise in pond-water nutrient concentrations. Here, we performed a nutrient-amendment experiment to examine short-term (≤72 h) nutrient uptake and identify the consumers of the added nutrients (planktonic vs. benthic communities). Microcosms (1 L) with and without sediment were experimentally amended with inorganic nitrogen (nitrate, ammonium) with and without phosphate. Amended nitrate and ammonium concentrations remained high in microcosms without sediments, and phytoplankton biomass did not change relative to the un-amended control. However, phosphate concentration declined significantly in microcosms without sediment, resulting in significant increase of phytoplankton biomass after 72 h. In the presence of sediment, amended nutrients were rapidly removed from the water, stimulating benthic algal biomass when phosphate was co-amended with ammonium or nitrate. Phytoplankton biomass was significantly elevated in microcosms with sediment compared to those without sediment, regardless of whether nutrients were amended or not, indicating that sediment and associated benthic biofilm stimulate phytoplankton growth, likely via supply of nutrients to the overlying water column. A key outcome of the experiment is that pulsed nutrients were taken up rapidly and primarily by the benthic community. Findings suggest that shallow ponds in the HBL are capable of rapidly consuming pulsed nutrient supplies, as may occur due to hydroclimatic events, climate warming and other disturbances.

Grazing rates of crustacean zooplankton communities on intact phytoplankton communities in Canadian Subarctic lakes and ponds

Zooplankton grazing can potentially affect the biomass and composition of phytoplankton communities directly and indirectly. Low chlorophyll a concentration for a given TP concentration and simplified fishless food webs lead to the expectation that zooplankton community grazing rates are high in Subarctic regions; however, zooplankton community grazing rates have not been determined for Subarctic lakes/ponds. We estimated zooplankton community grazing rates on phytoplankton in 12 lakes and ponds in Wapusk National Park, Canada using a microcosm grazing experiment. Lakes and ponds differed in zooplankton taxonomic composition, Chl-a concentration, and zooplankton biomass. We found that the grazing rates on the total chlorophyll a (GRTotal) ranged 0–13.7% grazed per day and the grazing rates on the edible (<30 μm, GR<30) chlorophyll a was 0 to 16.7% per day. GRTotal increased with lake Daphnia and cladoceran biomass, as did GR<30, which also had a negative relationship with the total in-lake Chl-a. The calculated zooplankton grazing rates were within the range found for larger, temperate lakes.

Carbon quantity defines productivity while its quality defines community composition of bacterioplankton in subarctic ponds

Bacterial communities in 16 oligotrophic ponds in Kilpisjarvi, subarctic Finland, were studied to test the hypothesis that dissolved organic carbon (DOC) quantity and quality differently influence bacterioplankton. The ponds were located below and above treeline at 600 m a.s.l., with 2–4 fold higher concentration of DOC below treeline. The concentration of DOC changed during the open-water season with highest values measured in mid-summer. Bacterial production, abundance, biomass were highest in mid-summer and correlated positively with the concentration of DOC. Quality indices of DOC showed that spring differed from the rest of the season. Highest specific UV-absorbance (SUVA) and humification index (HI), ratio a250/a265 and lowest fluorescence index (FI) were found during spring compared to summer and autumn, possibly indicating higher relative importance of allochthonous carbon during spring and a seasonal effect of photo-oxidation. According to Length Heterogeneity Polymerase Chain Reaction (LH-PCR) analyses, bacterial communities in spring were significantly different from those later in the season, possible due to the introduction of terrestrial bacteria associated with higher molecular weight material in spring DOC. Comparison between ponds situated above and below treeline revealed that bacteria were more abundant and productive at lower altitudes, which is probably connected to higher concentrations of DOC. The results also suggest that increased temperature and precipitation induced by global change and consequent higher allochthonous DOC runoff from the catchment could have a strong impact on biomass, productivity and community composition of micro-organisms in subarctic ponds and lakes.

Food sources and lipid retention of zooplankton in subarctic ponds

SUMMARY 1. Subarctic ponds are seasonal aquatic habitats subject to short summers but often have surprisingly numerous planktonic consumers relative to phytoplankton productivity. Because subarctic ponds have low pelagic productivity but a high biomass of benthic algae, we hypothesised that benthic mats provide a complementary and important food source for the zooplankton. To test this, we used a combination of fatty acid and stable isotope analyses to evaluate the nutritional content of benthic and pelagic food and their contributions to the diets of crustacean zooplankton in 10 Finnish subarctic ponds. 2. Benthic mats and seston differed significantly in total lipids, with seston (62.5 l gm g )1 ) having approximately eight times higher total lipid concentrations than benthic mats (7.0 l gm g )1 ). Moreover, the two potential food sources differed in their lipid quality, with benthic organic matter completely lacking some nutritionally important polyunsaturated fatty acids (PUFA), most notably docosahexaenoic acid and arachidonic acid. 3. Zooplankton had higher PUFA concentrations (27–67 l gm g )1 ) than either of the food sources (mean benthic mats: 1.2 l gm g )1 ; mean seston: 9.9 l gm g )1 ), indicating that zooplankton metabolically regulate their accumulation of PUFA. In addition, when each pond was evaluated independently, the zooplankton was consistently more 13 C-depleted (d 13 C )20 to )33&) than seston ()23 to )29&) or benthic ()15 to )27&) food sources. In three ponds, a subset of the zooplankton (Eudiaptomus graciloides, Bosmina sp., Daphnia sp. and Branchinecta paludosa) showed evidence of feeding on both benthic and planktonic resources, whereas in most (seven out of 10) ponds the zooplankton appeared to feed primarily on plankton. 4. Our results indicate that pelagic primary production was consistently the principal food resource of most metazoans. While benthic mats were highly productive, they did not appear to be a major food source for zooplankton. The pond zooplankton, faced by strong seasonal food limitation, acquires particular dietary elements selectively.