Macrophyte development and resuspension regulate the photosynthesis and production of benthic microalgae

The effect of macrophyte growth on microbenthic photosynthetic activity was studied in two large enclosures situated in a shallow, eutrophic lake. Macrophytes were allowed to develop stands of 100% coverage in one enclosure whereas they were harvested at emergence in the other. Although less than 10% of the incident light reached the benthic microphytes below the macrophytes at mid-summer, when the macrophytes reached their maximum coverage, the seasonal productivity (April-October) of the microbenthic community was still 355 g C m(-2) corresponding to 65% of the productivity in the enclosure without macrophytes. Although the light attenuation by the macrophytes had a strong negative effect on microbenthic photosynthesis, the negative effect was partly balanced by increased water transparency caused by increased grazing on the phytoplankton, and the shelter provided by the plants also resulted in less resuspension. Analysis with microsensors for oxygen and scalar irradiance showed that the capacity for photosynthesis was evenly distributed throughout the uppermost 3 mm of the sediment and in the approximately 3-mm flocculent layer covering the sediment. Microbenthic photosynthesis seemed primarily limited by light. The microsensor analysis also demonstrated how conventional oxygen exchange experiments underestimate the true photosynthetic rates and indicated that more realistic rates might be obtained by measuring oxygen exchange if the exchange is facilitated by vigorous stirring.