Short-term and seasonal dynamics of bacterial biomass production and amino acid turnover in the water column of an intertidal ecosystem, the Wadden Sea

Intertidal flat systems exhibit systematic temporal patterns of suspended particulate matter (SPM) and associated properties (particulate organic carbon, chlorophyll) with enhanced concentrations towards the current velocity maximum (CVM) and reduced concentrations towards slack water. It is unclear how these tidal patterns are reflected in organic matter processing by particle-associated (PA) and free-living (FL) heterotrophic bacteria. Therefore, we studied the abundance of PA and FL bacteria, bacterial biomass production (BP) and turnover of dissolved free amino acids (DFAA) during tidal cycles in a 13 to 16 m deep back-barrier intertidal flat system of the East Frisian Wadden Sea, Germany, in January, April, July and November 2005. Pronounced tidal patterns of microbial parameters were found with distinct differences around the CVM and at slack water. Tidal patterns varied seasonally with higher BP rates and bacterial generation times of 1 to 2 d in April and July. There was a trend of enhanced rates of BP from surface to bottom in April and July with an increasing significance of PA bacteria towards the bottom. Very high depth-integrated daily rates of BP and oxygen undersaturation suggest that this system is net-heterotrophic. A normalization procedure revealed systematic tidal patterns of most parameters irrespective of the seasonal situation. Our findings thus demonstrate the presence of tidal patterns of parameters reflecting microbial processes and thus complement previous findings of tidal patterns of SPM-associated properties. The tidally varying microbial activities indicate that the rhythmic tidal currents promote organic matter decomposition by heterotrophic bacteria in intertidal flat systems.