Several studies have shown that submerged macrophytes provide a refuge for zooplankton against fish predation, whereas the role of emergent and floating-leaved species, which are often dominant in eutrophic turbid lakes, is far less investigated.

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 lg mg )1 ) having approximately eight times higher total lipid concentrations than benthic mats (7.0 lg mg )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 lg mg )1 ) than either of the food sources (mean benthic mats: 1.2 lg mg )1 ; mean seston: 9.9 lg mg )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.

Summary 1. Shallow arctic lakes and ponds have simple and short food webs, but large uncertainties remain about benthic–pelagic links in these systems. We tested whether organic matter of benthic origin supports zooplankton biomass in a pond in NE Greenland, using stable isotope analysis of carbon and nitrogen in the pond itself and in a 13C‐enrichment enclosure experiment. In the latter, we manipulated the carbon isotope signature of benthic algae to enhance its isotopic discrimination from other potential food sources for zooplankton. 2. The cladoceran Daphnia middendorffiana responded to the 13C‐enrichment of benthic mats with progressively increasing δ13C values, suggesting benthic feeding. Stable isotope analysis also pointed towards a negligible contribution of terrestrial carbon to the diet of D. middendorffiana. This agreed with the apparent dominance of autochthonous dissolved organic matter in the pond revealed by analysis of coloured dissolved organic matter. 3. Daily net production by phytoplankton in the pond (18 mg C m−2 day−1) could satisfy only up to half of the calculated minimum energy requirements of D. middendorffiana (35 mg C m−2 day−1), whereas benthic primary production alone (145 mg C m−2 day−1) was more than sufficient. 4. Our findings highlight benthic primary production as a major dietary source for D. middendorffiana in this system and suggest that benthic organic matter may play a key role in sustaining pelagic secondary production in such nutrient‐limited high arctic ponds.

Bacterial communities in 16 oligotrophic ponds in Kilpisjärvi, 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 midsummer. 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.

An influx of terrestrial dissolved organic carbon (DOC) into freshwater habitats can regulate a range of ecosystem characteristics, from water clarity to productivity. To understand the extent to which DOC can regulate ecosystem functioning, we conducted a survey to determine the source of DOC in low Arctic ponds close to the Arctic Circle (Kangerlussuaq, Greenland), including its role in food web dynamics. We used a multiple element (carbon, nitrogen, and hydrogen) stable isotope approach to examine the proportional contribution of different sources to aquatic consumers in nine arctic ponds that spanned a broad gradient of DOC (6.6-60.1 mg L-1). Our results show that benthic and pelagic primary production decreased along a gradient of increasing DOC content. Additionally, the changes in the organic matter pool with increasing DOC translated into changes in consumer resource use. We found significant differences in resource use between species. All consumers relied on benthic autotrophic material when DOC was low; but when DOC was high some consumers changed their diet. Collectively, our findings demonstrate how the concentration of DOC influences aquatic production and our study can be used as a baseline to predict how the aquatic food web may respond to regionally changing DOC concentrations.