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

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, P B 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.

Decoupling of nutrient and grazer impacts on a benthic estuarine diatom assemblage

“…Microphytobenthos produce oxygen (Dalsgaard, 2003), fix large amounts of carbon (Gerbersdorf et al, 2005), drive benthic–pelagic nutrient exchanges (Ferguson et al, 2004), and provide trophic support (Sullivan and Moncreiff, 1990). Microphytobenthic communities are diverse at the species and group levels (Sullivan, 1978; Cahoon, 1999), but are often treated as a “black box,” where benthic microalgae are considered as a whole and the characteristics of individual key groups such as diatoms, green algae, and cyanobacteria are not evaluated individually.…”

Microphytobenthos in Shallow Arctic Lakes: Fine-Scale Depth Distribution of Chlorophylla, Radiocarbon Assimilation, Irradiance, and Dissolved O2

“…Lower and higher K d values are associated with sandy and cohesive sediments, respectively (Kühl et al, 1994). Strong scattering in the former can result in both a peak light intensity at the surface of up to 200% of incident values and deeper light penetration in sediment, while a dominance by absorption in organic sediments leads to a peak irradiance at the sediment surface and more rapid down-core light attenuation (Gerbersdorf et al, 2005). Our K d values of 1.2-2.1 mm ‫1מ‬ compare most favorably with those for sandy sediments, but shapes of profiles of photon scalar irradiance more closely resemble those for cohesive sediments.…”

“…High water content of the flocculent organic sediment at our sites (Table 2) is likely responsible for both low K d values and the monotonic decrease in irradiance with increasing depth below the sediment surface. Measurements of gross photosynthesis in cohesive marine sediments using O 2 microelectrodes indicate that the majority of the photosynthesis occurs in the surface 0.4 mm (Underwood and Kromkamp, 1999), while photosynthetic activity may extend to 2 mm in sandy sediment (Gerbersdorf et al, 2005). Although H 14 CO 3 ‫מ‬ -derived productivity estimates for intact cores must be cautiously interpreted due to vertical gradients of the radiocarbon label (Revsbech et al, 1981), our experiments are in qualitative agreement with these electrochemical marine observations that show a rapid decline in photosynthesis with increasing sediment depth.…”

Vertical migration of phytoplankton in coastal waters with different UVR transparency

“…A maximum of phytoplankton abundance was defined as a minimum of three subsequent layers/depths with Chlorophyll a concentrations exceeding the average Chlorophyll a concentration of the total vertical depth profile by more than 19% for Helgoland and 9% for Hiddensee measurements. The lower threshold for Hiddensee was at first chosen under the assumption that the higher trophic status at this water body [31] may result in shifts of the phytoplankton communities towards non-mobile species; as could be shown by Wasmund and Uhlig [32] as well as Sagert et al [33] for the German Baltic Coast. In fact, the proportion of motile flagellates has been much higher for Helgoland waters as compared to Hiddensee.…”