No abstract

No abstract

'~e p a r t m e n t of Marine Botany and *~e p a r t m e n t

ABSTRACT-Eutrophication has increased the amount of drifting ephemeral algae in shallow coastal bays. These floating algal mats can b e 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 con~position of microalgae and meiofauna, biomass of bacteria, chlorophyll a content, pigment ratios. primdrl; productivity, sediment oxygen microprofiles, and inorganic nutrient fluxes. Occasional held measurements of light, oxygen, and inorganic nutnents above and belo\\l 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 oi microorganisms were found. Either no quantitative effects were found at all, or an initial stimulus of rnicroalgal 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 greylsh. 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 sitir 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 dlatom-dominated microbial mats for adaptlng to life below another algal mat. Although light-induced orientation and vertical movement of diatoms appear to play a major role in thls acclimation, the role of other mechanisms, such as hetel-otrophic nutrition, should be examined.

Intact natural microbial mats collected from a shallow microtidal bay were incubated in the presence or absence of ambient ultraviolet-B radiation (UVBR) in an outdoor flow-through system for 2 and 6 wk, respectively. The microbial mats were cohesive and dominated by a large, motile diatom species, Gyrosigma balticum (Ehrenberg) Rabenhorst. The variables studied included biomass, composition of microalgae and meiofauna, bacterial biomass, pigment composition and presence of UV-absorbing compounds. Carbon assimilation and sediment oxygen microprofiles were measured as indicators of photosynthetic and respiratory activity. No significant effects were found on any of the measured variables when natural UVBR was excluded, suggesting that ambient UVBR did not exert any strong selective pressure on the particular type of microbial mat studied Biochemical screening by UV-absorbing compounds did not appear to be of major importance for avoiding UVBR damage. Visual observations, and to some extent oxygen profiles, indicated downward migration of the dominant diatom species as a response to high light levels. This may have been an important strategy also to avoid exposure to UVBR. Thus, vertical migration of benthic diatoms appears to be a factor that should be further investigated in relation to effects of both present and enhanced levels of UVBR.