Interactions Between Benthic Macroalgal and Microalgal Mats

Benthic primary producers play a key role in regulating carbon and nutrient turnover in shallow-water coastal areas. Macroalgal (MA) mats, which persist from months to over a year, consist of fast-growing filamentous or sheet-like algae on the sediment or floating at the water surface. Benthic microalgal mats (microphytobenthos, MPB), which occur throughout the year, are mostly dominated by diatoms and confined to the photic zone of the sediment. The two mats interact, directly and indirectly, through their impact on light, oxygen, and nutrient conditions. While shading by dense MA mats decreases MPB production, there are shade-adapted MPB that co-exist with MA. Oxygen deficiency below dense and decomposing MA affects the sediment community and increases nutrient efflux from the sediment, a feedback mechanism that favours MA growth. Both mats are temporary sinks for inorganic nutrients and reduce the efflux of remineralized nutrients from the sediment, thereby decoupling benthic and water-column nutrient turnover. Despite large differences in biomass, the quantitative role of both mats for nitrogen assimilation can be similar, making turnover time of algae-bound nutrients a key factor in nutrient retention. The MA-MPB coupling cannot be described by a single model, but rather by different models describing scenarios of MA occupying the water column or the sediment surface. Support was found for the general eutrophication models, predicting that benthic production will decrease with increasing nutrient levels. The impact on MPB may be slower than for seagrasses, and MPB, resistant to anoxia, may promote benthic recovery by rapidly re-oxygenating the sediment surface.