Primary production in two shallow lakes with contrasting plant form dominance: A paradox of enrichment?

We estimated total lake plant biomass and primary net production in two shallow Swedish lakes that differ in nutrient loading and plant form dominance. In clearwater Lake Krankesjon (10 mu g chlorophyll a L-1), submerged macrophytes contributed more than phytoplankton and epiphyton to the estimated plant biomass. Estimated net primary production during May to September was 90-130, 1.2, and 14 g C m(-2) for phytoplankton, epiphyton, and submerged macrophytes, respectively. In turbid Lake Borringesjon (60-80 mu g chlorophyll a L-1), primary production by submerged macrophytes and periphyton was negligible. Although gross primary production of phytoplankton was high close to the water surface, estimated areal net primary production during May to September was low, -40 to +25 g C m(-2), as a result of self-shading and high respiration. Grazing pressure from zooplankton rarely exceeded 15% d(-1) in both lakes, indicating that phytoplankton production was not limited by grazing. Low gross epiphyton production could result from high grazing by macroinvertebrates and thus higher trophic transfer efficiency through the benthic than through the pelagic food web. Provided that conditions in Lake Borringesjon reflect previous turbid state conditions in Lake Krankesjon, our results may explain why a shift to a clearwater state was followed by increased biomass of higher trophic levels. Our results also support the paradox of enrichment hypothesis, which predicts lower productivity at high nutrient loading. Contrary to former investigations, we found lower production at a higher nutrient loading already at the trophic level of primary producers.