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

Abstract

The effect of macrophyte growth on microbenthicphotosynthetic activity was studied in two largeenclosures situated in a shallow, eutrophic lake.Macrophytes were allowed to develop stands of 100%coverage in one enclosure whereas they were harvestedat emergence in the other. Although less than 10% ofthe incident light reached the benthic microphytesbelow the macrophytes at mid-summer, when themacrophytes reached their maximum coverage, theseasonal productivity (April–October) of themicrobenthic community was still 355 g C m⁻²corresponding to 65% of the productivity in theenclosure without macrophytes. Although the lightattenuation by the macrophytes had a strong negativeeffect on microbenthic photosynthesis, the negativeeffect was partly balanced by increased watertransparency caused by increased grazing on thephytoplankton, and the shelter provided by the plantsalso resulted in less resuspension. Analysis withmicrosensors for oxygen and scalar irradiance showedthat the capacity for photosynthesis was evenlydistributed throughout the uppermost 3 mm of thesediment and in the approximately 3-mm flocculent layercovering the sediment. Microbenthic photosynthesisseemed primarily limited by light. The microsensoranalysis also demonstrated how conventional oxygenexchange experiments underestimate the truephotosynthetic rates and indicated that more realisticrates might be obtained by measuring oxygen exchangeif the exchange is facilitated by vigorousstirring.