Impact of Macrofaunal Communities on the Coastal Filter Function in the Bay of Gdansk, Baltic Sea

Abstract

During three cruises to the Bay of Gdansk, Baltic Sea, the fauna, porewater and bottom water were sampled at stations parallel to the shore and along a transect offshore. Diffusive porewater fluxes were calculated and related to the total net fluxes (TNF) of nutrients. The TNF comprise all nutrients that reach the bottom water from the sediment including diffusive nutrient efflux, discharge from macrozoobenthos and microbial activity. They were determined during in situ incubations using a benthic chamber lander, which is rarely done in coastal research. The lander restricts the physical influence of currents and waves on the sediments and only allows nutrient fluxes due to bioturbation by natural communities. Strong benthic-pelagic coupling in the shallow coastal zone suggested a crucial filter function for the bioturbated coastal sediments, which are separated from muddy deep sediments with little or no fauna at a depth of 50 m; in between is a small intermediate zone. While diffusive fluxes were highest at intermediate and offshore stations, TNF were highest at sandy coastal stations, where reservoirs of dissolved nutrients were small and sediments almost devoid of organic material. The greatest impact of macrofauna on sedimentary fluxes was found at stations whose communities were dominated by deep-burrowing polychaetes. The largest TNF were measured directly at the mouth of the Vistula River, where riverine food and nutrients supplies were highest. Macrofauna communities and sediment variables can thus serve as descriptive indicator to estimate the extent of the coastal filter. Finally, based on the total areal size of the different sediment types, annual efflux for the complete coastal zone of the Gdansk Bay was estimated to be 6.9 kt N, 19 kt Si, and 0.9 kt P. Compared to the muddy offshore area, which is twice as large, these amounts were similar for P and threefold higher for N and Si.

In Search of a Field-Based Relationship Between Benthic Macrofauna and Biogeochemistry in a Modern Brackish Coastal Sea

“…Gammal et al (2017) emphasized the increase of solute effluxes of phosphate and ammonium from sediments with decreased macrofaunal activity induced by coastal hypoxia in the western Gulf of Finland. The recent study in the Bay of Gdansk by Thoms et al (2018) determined the greatest impact of macrofauna on sedimentary fluxes at stations where communities were dominated by deep-burrowing polychaetes. The authors suggested that these communities can serve as a descriptive indicator to estimate the extent of the coastal filter, where organic matter is reworked and relatively large amounts of nutrients (NH + 4 , NO − 3 , PO 3− 4 , SiO 2 ) are released into the bottom water.…”

Lagrangian Residence Time in the Bay of Gdańsk, Baltic Sea

“…The 50-m depth contour in the BG (Figure 3) separates near-coastal sandy sediments from offshore muddy sediments (Thoms et al, 2018). This differentiation also appears in the LRT, which was longer in the coastal area shallower than 50 m water depth than in the northern offshore area.…”

“…The longer surface-water LRT in the near-coastal area may be conducive to the sedimentation of organic matter deriving from primary production on the near-coastal seafloor. This would fuel benthic nutrient turnover, especially nutrient retention via fluxes (Thoms et al, 2018) and nitrification (Bartl et al, 2019), in turn facilitating permanent nutrient-removal processes such as denitrification and burial. Thus, the surface-water LRT may have an indirect impact on the permanent removal of nutrients from this coastal system, which functions on time scales much longer than the mean water residence time (Asmala et al, 2017).…”

“…The high and unbalanced Redfield ratios in the coastal zone may alter nitrate cycling (Lunau et al, 2012). To understand the efficiency of nitrate removal, nutrient retention capacity and the complexity of the coastal filter function in the BG (Thoms et al, 2018;Bartl et al, 2019) requires knowledge of the water residence time. Based on the LOICZ (Land Ocean Interactions in the Coastal Zone) model of Gordon et al (1996); Witek et al (2003) computed a flushing time of 15 days in the BG, which they named "water exchange time" or "water residence time".…”

The Influence of Sediment‐Derived Dissolved Organic Matter in the Vistula River Estuary/Gulf of Gdansk

“…Oceanographic data can be found at the Baltic Environmental Database http:// www.balticnest.org/bed. Sediment data can be found in the supplemental information of Thoms et al (2018). This work was supported by the COCOA project under the BONUS research program funded by the European Commission; the Danish Research Council for Independent Research (DFF-1323-00336); and the German Federal Ministry of Science and Education, BMBF (03F0683A).…”