Role of Bacteria in the Carbon and Nitrogen Flow between Water‐Column and Sediment in a Shallow Marine Bay (Bay of Piran, Northern Adriatic Sea)

Abstract. The interdependences between phytoplankton standing crop, bacterial biomass and the dissolved organic matter (DOM) pool in the water column were investigated and related to sediment parameters in a shallow marine bay (Bay of Piran, Northern Adriatic Sea) over an annual cycle. Bacterioplankton density varied between 1–10 × 10⁵ cells ml^‐1, with lowest density observed in March corresponding to the low Chi a concentrations during this period. Generation times as determined by dialysis incubations ranged between 4h (June) and 82 h (March). Mean bacterial secondary production rates during summer were about 40 mg C m^‐1 d^‐1 and 5mg C m^‐3.d^‐1 during winter. With a short time lag, DOM concentrations followed the fluctuation in Chi a. Sediment oxygen demand measurements revealed a mean mineralization rate of about 260 mg C m^‐2 d^‐1 during summer and 100–200 mg C m^‐2 d^‐1 in winter. Sediment bacterial density varied between 10⁸ ‐ 10⁹ cells g (sediment dry wt)^‐1 in the top 5 cm sediment layer or, in terms of biomass, 4.3 g C m^‐l during summer and 0.6 g C m^‐2 during winter. Highest concentrations of DOM in pore waters were measured in September, coinciding with high rates of sediment oxygen demand.