Inter-Research
Aquatic Microbial Ecology

ABSTRACT: Eutrophication has increased the amount of drifting ephemeral algae in shallow coastal bays. These floating algal mats can be expected to change the light climate of benthic primary producers and to induce oxygen deficiency, which may increase nutrient release from the sediment. The impact of healthy green algal mats, mainly Enteromorpha spp., on benthic microbial mats was studied in five 2 to 6 wk outdoor experiments by incubating intact sediment below natural green algal mats from a microtidal bay on the west coast of Sweden. The microbial mats were cohesive and dominated by large motile diatoms, mainly Gyrosigma balticum. Estimates of response were based on biomass and composition of microalgae and meiofauna, biomass of bacteria, chlorophyll a content, pigment ratios, primary productivity, sediment oxygen microprofiles, and inorganic nutrient fluxes. Occasional field measurements of light, oxygen, and inorganic nutrients above and below green algal mats were made. The results suggest that benthic diatom-dominated microbial mats need not be outcompeted by average amounts of healthy green algae during the growth season. Despite strongly reduced light below the green algae, no adverse effects on the biomass of microorganisms were found. Either no quantitative effects were found at all, or an initial stimulus of microalgal biomass was observed. The only consistent pattern reflecting treatment was that the sediment surface below the green algae became dark brown, while cores with no green algal cover became greyish. This appeared to be caused by a different vertical position of diatoms within the sediment. Sediment oxygen profiles indicated a higher photosynthetic efficiency below the green algal mats, indicating light acclimation of the microalgae. It is suggested that shade adaptation and sediment nutrient supply enable the diatoms, and other microbiota, to coexist with moderate amounts of healthy drift algae. Both in situ and in the experiments, algal photosynthesis kept oxygen concentrations above critical values, even at night, thereby preventing redox-related nutrient outflux from the sediment. The results hint at some not yet fully understood mechanisms of diatom-dominated microbial mats for adapting to life below another algal mat. Although light-induced orientation and vertical movement of diatoms appear to play a major role in this acclimation, the role of other mechanisms, such as heterotrophic nutrition, should be examined.

KEY WORDS: Sediment . Diatoms . Meiofauna . Chlorophyll . Motility . Light . Nutrients . Oxygen . Gyrosigma

Published in AME Vol. 10, No. 2 (1996) on May 9
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 1996