Biomass patterns in boreal-subarctic lake food webs along gradients of light and nutrients

There is large natural variation in light and nutrient conditions across lakes. In the boreal-subarctic region most lakes are small, shallow and nutrient poor. In such lakes there is often sufficient light to support primary production at the lake bottom. An expectation for the future is that colored dissolved organic matter (cDOM) of terrestrial origin will increase in these lakes. cDOM depresses the underwater light climate but is often associated with elevated pelagic nutrient concentrations. A dynamical model of a coupled benthic-pelagic food web was explored for how lake ecosystems might respond to altered light and nutrient regimes. The model predicts that mobile carnivores (fish) control grazers and release primary producers from grazing pressure. Primary producers are therefore limited by their resources and cross-habitat interactions are dominated by spatially asymmetric competition for light and nutrients. At high light and low nutrient supply benthic algae out-compete pelagic algae for nutrients diffusing from the sediment, whereas pelagic algae shade out benthic algae at lower light and/or higher nutrient supply. Biomass patterns of benthic and pelagic consumers follow the patterns of primary production. In contrast, habitat coupling through carnivore movement has only a weak impact on biomass patterns in the model food web. Model predictions were compared with data from boreal-subarctic lakes covering a broad range of cDOM concentrations. In agreement with model expectations the following relationships with increasing light attenuation were observed: benthic primary and secondary production decreased, pelagic primary production showed a unimodal trend, and pelagic nutrient concentrations as well as the proportion of fish feeding in the pelagic habitat increased. As a consequence, both primary and fish production were negatively related to pelagic nutrient concentrations across lakes. In a comparative study of boreal-subarctic lakes covering a broad range of cDOM concentrations, a similar negative relationship was found between pelagic total nutrient concentrations and the biomass of epilithic algae. This was surprising, because epilithon cannot access nutrients from the sediment. Patterns in epilithon biomass were largely driven by nitrogen fixing cyanobacteria, which in turn were positively related to light supply. The data suggest that nitrogen fixing autotrophs may have a competitive advantage over other epilithic primary producers in low-cDOM, low-nutrient, high-light environments, and that patterns in epilithic biomass, nutrient sequestration and elemental stoichiometry depend upon which functional group is dominant in the epilithic biofilm