Continuous and High Transport of Particles and Solutes by Benthos in Coastal Eutrophic Sediments of the Pomeranian Bay

Abstract

We present results on bioirrigation and reworking of sediments by benthic macrofauna in sediments of the Pomeranian Bay (southern Baltic Sea), that were obtained ∼22 years ago. The investigation took place at four stations ranging from 9 to 19 m water depth, which we observed between 1993 and 1995 over the course of 30 months. In order to assess exchange of solutes across and particles from the sedimentwater interface with the underlying sediments, we used bromide as ex situ tracer for bioirrigation and chlorophyll a equivalents as in situ tracer for particle reworking. Using models to interpret tracer distributions in the sediment, we compare the magnitudes of small scale, diffusion-like, versus relatively large-scale, non-local, transport modes. Our results indicate a spatial differentiation of the bay: the coastal station most heavily impacted by eutrophication close to the Oder River mouth showed medium reworking and intense bioirrigation combined with lower chlorophyll concentrations throughout the sediment. This contrasts with high surface pigment concentrations at the shallow Oder Bank station, indicating benthic primary production and intense particle mixing. Medium local particle reworking at the northwestern station and medium mean solute transport rates characterized the two deeper stations in the northern Bay. The bromide tracer experiments, which exclusively depict animal activity, showed significant biological solute transport to ∼10 cm sediment depth within 3 days. Non-biological transport mechanisms in the field (resuspension, fishing activity, and pore water advection) might additionally affect the in situ tracer chlorophyll a-depth distributions. This tracer, too, indicates mixing within the uppermost 10 cm of the sediment within ∼2-3 months. In general, experimentally obtained solute transport constants were higher than most values reported previously (surface α: 155 year −1) and particle reworking was at the high end of reported values as well. Thus, benthic fauna is responsible for an intense bioturbation in Pomeranian Bay. Specifically, high bioirrigation rates were associated with high density and biomass of deep burrowing polychaetes. Recently published rates of particle reworking in the same area are on the same order of magnitude as ours obtained two decades ago. This finding is consistent with the species composition which generally remained the same.

Benthic Macrofauna Community Bioirrigation Potential (BIPc): Regional Map and Utility Validation for the South-Western Baltic Sea

“…Published global estimates, such as those based on relationships between bottom water oxygen and nitrogen loss [ 46 ], fluxes of phosphorus [ 47 ] and biogenic silica [ 48 ] appeared to be too coarse in spatial resolution when compared to our investigation. Apart from negligible sampling size, the weak relationship between estimated BIPc values in this study and irrigation volumes from Powilleit and Forster [ 43 ] (based on a handful of stations) could be associated with the poor performance of the index, or also with experimental artefacts arising during the measurements. The differences in association for different sediment layers indicated that correspondence between the Br transport and fauna is blurred close the sediment–water interface where, in addition to bioirrigation, molecular diffusion drives tracer fluxes most.…”

“…Furthermore, the spatial pattern of the tracer in pore waters also showed a positive association with BIPc values. A correlation of index with bioirrigation constants calculated for various depth intervals reported in Powilleit and Forster [ 43 ] was not significant, but this is not surprising due to the very low sample size. The strength of linear association was higher for deeper sediment depth intervals.…”

“…There are some comparisons indicating that the patterns shown by BIPc distribution may not be unrealistic. Powilleit and Forster [ 43 ] reported bioturbation and bioirrigation rates from Pommeranian Bight. Reported bioirrigation indicated the highest rates ever measured using the tracer NaBr in this marine area, supporting the findings in Figure 4 a.…”

Influence of Manila clam aquaculture on rates and partitioning of organic carbon oxidation in sediment of Keunso Bay, Yellow Sea

“…Accumulation of organic matter and intense sediment reworking (i.e. physical mixing) can have a substantial influence on sediment biogeochemistry (Welsh 2003, McKindsey et al 2011, Kristensen et al 2012, Wendelboe et al 2013, Koo & Seo 2017, Powilleit & Forster 2018.…”