Terrestrial organic material (t-OM) can subsidize lake food webs indirectly via incorporation of dissolved t-OM by bacteria and subsequent transfer to higher trophic levels or directly through metazoan consumption of particulate t-OM (t-POM). We tested the effects of peat layer t-POM on Daphnia galeata performance. A pure t-POM diet could not sustain survival, growth, and reproduction of D. galeata. Mixtures of heterotrophic bacteria (Pseudomonas sp.) and phytoplankton (Rhodomonas lacustris) gave higher survival, growth, and reproduction than mixtures of t-POM and Rhodomonas. Daphnids performed best when feeding on pure Rhodomonas diets. Quantification of phosphorus (P) and essential biochemicals (i.e., fatty acids) revealed that Rhodomonas had the highest amounts of all these components. Pseudomonas, while rich in P, contained few essential fatty acids, and t-POM had low concentrations of both P and fatty acids. We therefore suggest that the poor food quality of t-POM in our experiment was due to its suboptimal mineral and biochemical composition and that a substantial proportion of high-quality phytoplankton is necessary to sustain zooplankton biomass.

1. Bacteria can be an important resource for zooplankton production in aquatic food webs, although the degree to which bacteria sustain zooplankton growth and reproduction is not clear. We performed a growth experiment with Daphnia galeata feeding on different ratios of P-replete Pseudomonas and Rhodomonas, ranging from a 100% bacterial to a 100% algal diet. 2. A pure bacterial diet did not support survival, growth or reproduction of D. galeata. While a 20% share of Rhodomonas in the food allowed survival of daphniids, the occurrence of offspring on a 50% algal diet indicated that the threshold for successful reproduction was between those two proportions of algal food. Increasing the proportion of the alga further increased growth and reproductive output, indicating that Rhodomonas was a higher-quality food than Pseudomonas. A subsequent labelling experiment demonstrated that D. galeata incorporated phosphorus fromPseudomonas and Rhodomonas with similar efficiency, whereas carbon was incorporated more efficiently from Pseudomonas than from Rhodomonas. 4. Hence, we hypothesise that inadequate levels of essential biochemicals in pure bacterial diets led to decreased Daphnia performance. Concentrations of fatty acids in general, and especially of polyunsaturated fatty acids, were much lower in Pseudomonas than in Rhodomonas. This difference could explain the different growth and reproduction responses, although limitation by other essential biochemicals (e.g. sterols) cannot be ruled out. 5. Hence, where they dominate, bacteria may provide a significant part of the elemental flux to species feeding higher in the food web on the short term. However, the performance of consumers may be constrained by essential biochemicals.

Effects of wave surge on growth and nutrient uptake in the green seaweed Ulva pertusa were measured in small tanks fed natural seawater via a dump bucket that could be either locked in place to deliver water continuously, or set to tip at particular volumes. Rates of bulk water flow through the tanks were low (<13 mm s -1 ), as may be typical of highly sheltered marine and estuarine environments. At the maximum bulk flow rates tested, wave surge increased growth in wet mass 1.6-fold. Wave surge increased the rate of ammonium uptake 1.5-fold over the full range of bulk flow rates tested (0.2 to 2.9 mm s -1 ). In winter, when light probably limited U. pertusa in our outdoor cultures, growth in wet mass was relatively unaffected by wave surge and completely unaffected by the experimental addition of nutrients (N and P). However, in summer, when ambient nutrients probably limited growth, wave surge increased growth in wet mass at low (0.21 mm s -1 ) and high (4.06 mm s -1 ) bulk flow rates, and nutrients increased growth rates at low but not high bulk flow rates. Together, these results indicate that the effect of wave surge on U. pertusa at low bulk flow rates (ca. < 2 mm s -1 ) is to enhance the seaweed's access to nutrients. However, at higher bulk flow rates wave surge has a different effect, which we speculate is due either to provision of a micronutrient or removal of an inhibitory substance from the concentration boundary layer. Our work demonstrates how the lack of small-scale water movement in sheltered habitats can constrain seaweed growth.

This study demonstrates that clear and humic freshwater pelagic communities respond differently to the same environmental stressors, i.e. nutrient and light availability. Thus, effects on humic communities cannot be generalized from existing knowledge about these environmental stressors on clear water communities. Small humic lakes are the most numerous type of lake in the boreal zone, but little is known about how these lakes will respond to increased inflows of nutrients and terrestrial dissolved organic C (t-DOC) due to climate change and increased human impacts. Therefore, we compared the effects of nutrient addition and light availability on pelagic humic and clear water lake communities in a mesocosm experiment. When nutrients were added, phytoplankton production (PPr) increased in both communities, but pelagic energy mobilization (PEM) and bacterial production (BP) only increased in the humic community. At low light conditions, the addition of nutrients led to increased PPr only in the humic community, suggesting that, in contrast to the clear water community, humic phytoplankton were already adapted to lower ambient light levels. Low light significantly reduced PPr and PEM in the clear water community, but without reducing total zooplankton production, which resulted in a doubling of food web efficiency (FWE = total zooplankton production/PEM). However, total zooplankton production was not correlated with PEM, PPr, BP, PPr:BP or C:nutrient stoichiometry for either community type. Therefore, other factors such as food chain length, food quality, ultra-violet radiation or duration of the experiment, must have determined total zooplankton production and ultimately FWE.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.