Benthic Diatoms on Sheltered Coastal Soft Bottoms (Baltic Sea)—Seasonal Community Production and Respiration

Abstract

Benthic diatom communities dominate sheltered shallow inner coastal waters of the atidal Southern Baltic Sea. However, their photosynthetic oxygen production and respiratory oxygen consumption is rarely evaluated. In the Baltic Sea carbon budget benthic diatom communities are often not included, since phytoplankton is regarded as the main primary producer. Therefore, two wind-protected stations (2–49-cm depths) were investigated between July 2010 and April 2012 using undisturbed sediment cores in combination with planar oxygen optodes. We expected strong fluctuations in the biological activity parameters in the incubated cores over the course of the seasons. The sediment particles at both stations were dominated by fine sand with a median grain size of 131–138 µm exhibiting an angular shape with many edges, which were less mobile compared to exposed coastal sites of the Southern Baltic Sea. These sand grains inhabited dense communities of rather small epipsammic diatoms (<10 µm). Chlorophyll a as a biomass parameter for benthic diatoms fluctuated from 64.8 to 277.3-mg Chl. a m−2 sediment surface. The net primary production and respiration rates exhibited strong variations across the different months at both stations, ranging from 12.9 to 56.9 mg O2 m−2 h−1 and from −6.4 to −137.6 mg O2 m−2 h−1, respectively. From these data, a gross primary production of 13.4 to 59.5 mg C m−2 h−1 was calculated. The results presented confirmed strong seasonal changes (four-fold amplitude) for the activity parameters and, hence, provided important production biological information for sheltered sediments of the Southern Baltic Sea. These data clearly indicate that benthic diatoms, although often ignored until now, represent a key component in the primary production of these coastal habitats when compared to similar studies at other locations of the Baltic Sea and, hence, should be considered in any carbon budget model of this brackish water ecosystem.