Abstract. Shallow coastal waters in many regions are subject to nutrient enrichment.
Microphytobenthos (MPB) can account for much of the carbon (C) fixation in
these environments, depending on the depth of the water column, but the
effect of enhanced nutrient availability on the processing and fate of
MPB-derived C (MPB-C) is relatively unknown. In this study, MPB was labeled
(stable isotope enrichment) in situ using 13C-sodium bicarbonate. The
processing and fate of the newly fixed MPB-C was then traced using ex situ
incubations over 3.5 days under different concentrations of nutrients
(NH4+ and PO43-: ambient, 2× ambient, 5×
ambient, and 10× ambient). After 3.5 days, sediments incubated with
increased nutrient concentrations (amended treatments) had increased loss of
13C from sediment organic matter (OM) as a
portion of initial uptake (95 % remaining in ambient vs. 79–93 % for
amended treatments) and less 13C in MPB (52 % ambient, 26–49 %
amended), most likely reflecting increased turnover of MPB-derived C
supporting increased production of extracellular enzymes and storage
products. Loss of MPB-derived C to the water column via dissolved organic C
(DOC) was minimal regardless of treatment (0.4–0.6 %). Loss due to
respiration was more substantial, with effluxes of dissolved inorganic C
(DIC) increasing with additional nutrient availability (4 % ambient,
6.6–19.8 % amended). These shifts resulted in a decreased turnover time
for algal C (419 days ambient, 134–199 days amended). This suggests that
nutrient enrichment of estuaries may ultimately lead to decreased retention
of carbon within MPB-dominated sediments.
Response of epilithic microphytobenthos of the Western Baltic Sea to in situ experiments with nutrient enrichment