Microphytobenthic primary production in the Bodden estuaries, southern Baltic Sea, at two study sites differing in trophic status

摘要

ABSTRACT: Eutrophication in coastal areas has stimulated phytoplankton growth, sustaining a high biomass and leading to a shift in the underwater light field. With the significance of the microphytobenthos for oxygen supply and carbon budget of both benthic and pelagic habitats in mind, the possible effects of reduced light availability were investigated in the estuarine Bodden area (southern Baltic Sea) at 2 sites differing in trophic status—the eutrophic Kirr Bucht (KB) and the mesotrophic Rassower Strom (RS). Using for the first time microsensors in Bodden sediments, it was possible to visualize small-scale heterogeneity in the light regime, photosynthetic activity and oxygen penetration with high spatial and temporal resolution. Hence, differences at the 2 sites related to sediment characteristics (KB sandy, RS muddy), and photoautotrophic biomass (benthic chlorophyll a in the upper 1 cm, µg cm–3 = 11 to 48 at KB and 13 to 17 at RS) could be ruled out. Calculations of benthic primary production based solely on microelectrode measurements revealed substantial oxygen fluxes and carbon fixation rates at in situ light intensities at both study sites (e.g. gross primary production, GPP, mg C m–2 h–1 = 28 to 80 at KB and 3 to 36 at RS). The different combinations of water transparency (pelagic chlorophyll a, µg l–1 = 12 to 33 at KB and 1.3 to 4.5 at RS), light attenuation k (3.17 m–1 at KB, 0.61 m–1 at RS) and water depth (0.6 m at KB, 3.4 m at RA) have led to a similar light availability for benthic algae on the sediment surface at both study sites. Consequently, the benthic algae had comparable productivity at both sites, with maximum primary production, PB_max (mg C mg–1 chlorophyll a h–1) of 0.29 to 1.46 at KB and 0.17 to 1.63 at RS; and were adapted to rather low light conditions, with light saturation, E_k (µE m–2 s–1) of 22 to 152 at KB and 10 to 116 at RS). Varying with season, microphytobenthic photosynthetic activity accounted for 26 to 59 and 2 to 53% to the total primary production at the KB and RS, respectively, with the highest contribution in spring coincident with the most favourable light conditions at the sediment surface. With an annual average of about 37 and 30% (KB and RS, respectively), the contribution of the microphytobenthos to total production was significant and comparable at both study sites. Nevertheless, the higher trophic status at KB resulted in a change in the benthic microalgal community towards sedimentated phytoplankton species and had a negative impact on microphytobenthic primary production rates. This was estimated by calculating the gross primary production for varying water depths on the basis of the different water transparency at the 2 sites.