## 4 Discussion  
### Benthic-Pelagic Coupling: Nutrient Release from Sediment and Effects on the Plankton  
In previous work, with larger cosms run over eight months [19], we showed that HBCDD negatively affected the biomass of \"large\" (>8 mm) _L. halthica_ in the same size class as in this experiment. This in turn affected benthic-pelagic coupling, reducing the recirculation of nutrients from the sediment to the water. In the current study, no changes in _L. halthica_ biomass were observed, probably due to the short duration of this study. Trends in benthic and pelagic parameters were not linked to differences in \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**t6** & & & & **t13** & \\ \hline
**Correlated Variable** & \(r\) & \(p\) & **Correlated Variable** & \(r\) & \(p\) \\ \hline
**Benthic primary production** & & & & & \\ _Dolichospermum_ heterocysts & 0.670 & 0.001 & & & \\ PO4 & 0.657 & 0.002 & & no significant correlations & \\ _Melosira_ sp. & 0.627 & 0.003 & & & \\ _Dolichospermum_ filaments & 0.596 & 0.006 & & & \\ Chlorophyll a & 0.478 & 0.033 & & & \\ \hline
**Bivalve larvae** & & & & & \\ _Pelagic_ PP & 0.694 & 0.002 & & no significant correlations & \\ _PO4 & \(-\)0.631 & 0.007 & & & \\ _Dolichospermum_ heterocysts & 0.583 & 0.014 & & & \\ \hline
**Gastropod larvae** & & & **Gastropod larvae** & & \\ _PO4_ & 0.718 & 0.000 & Total meiofauna & 0.620 & 0.008 \\ Chlorophyll a & 0.511 & 0.021 & Chlorophyll a & \(-\)0.566 & 0.018 \\ _Dolichospermum_ heterocysts & 0.501 & 0.024 & _Dolichospermum_ heterocysts & \(-\)0.505 & 0.039 \\  & & & PO4 & 0.475 & 0.054 \\ \hline
**Hydrobidae** & & & & no data available & & \\ Total N & 0.449 & 0.047 & & & \\ \hline no data available & & & **Total _Linecola halthica_ biomass (gdw)** & & \\ _Melosira_ sp. & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of significant results from the multiple correlations of the benthic-pelagic parameters. \(r\) is the correlation coefficient, \(p\) is the level of significance. _L. balthica_ biomass between treatments, but to overall effects of temperature, and to a lesser degree HBCDD (see Section 4.3). It is well-known from freshwater systems that increased temperature causes the release of PO\({}_{4}\) from sediments [25; 26; 27]. The few studies that have been performed in marine ecosystems suggest the same (e.g., [28; 29; 30]). This PO\({}_{4}\) release is thought to occur due to increased microbial activity increasing the decomposition of organic matter at the sediment surface [31; 32; 33] and by consuming oxygen at the sediment-water interface [25], leading to a redox-mediated release of P [27]. Studies of anoxic marine benthic ecosystems also confirm that low oxygen conditions facilitate fluxes of nutrients from the sediment to the water [28; 30; 34]. Although overall oxygen levels in the water column remained high in this experiment, it is possible that local anoxia/hypoxia at the sediment surface occurred in the warm treatments. We did not measure benthic bacterial activity specifically in this study, but measurements of benthic primary productivity support the idea that benthic processes were higher at higher temperatures, particularly at t6. By t13, P levels in the sediment were probably quite low, leading to the reduction in benthic primary productivity at this time point. It should be noted that by adding organic matter to the cosms at t0, we also added a source of P, which potentially could have increased water PO\({}_{4}\) concentrations. However, we estimate that this addition was negligible compared to the P pool in the sediment. Calculations using measured TOC and C:P ratios in the added suspended organic matter and the sediment suggest that the organic matter provided c. 0.35 mgP per cosm compared c. 1200 mgP in the sediment.