## 4 Discussion
### Benthic-Pelagic Coupling: Benthic Species' Larvae in the Zooplankton  
Benthic species were also present in the water column as larvae; bivalve and gastropod larvae were identified as two of the planktonic taxa that were responsible for differences in the zooplankton community structure. Although bivalve larvae may survive in the water column for many weeks, their general increase in numbers in the warm treatments throughout the experiment suggests that these were released from adults in the cosms, rather than being present in the water column from the start. In this case, they were most probably _L. ballhica_ (10 adults present in each cosm), or possibly _C. glaucum_ (2 per cosm), both of which are known to spawn at the time of year that the experiment was performed (e.g., [39; 40]). There were no clear effects on adult _L. ballhica_ or _C. glaucum_ in terms of mortality or size, so the clear effects on their larvae must be either due to direct effects on adult spawning or larval survival, or indirect effects on the larvae related to foodweb dynamics. Drent showed that _L. ballhica_ larvae grew faster and developed more quickly at higher temperatures (experimental range 10-20 \({}^{\circ}\)C) [41], and several other studies have shown that spawning is triggered by the temperature rising to a critical level [42; 43], albeit at lower temperatures than in this experiment. Benthic primary production at t6 was also positively correlated with bivalve larval abundance at t13; adult _L. ballhica_ may have been able to spawn more successfully in the warmer treatments by t13 due to increased food levels on the sediment surface. _L. ballhica_ larvae are thought to feed on small unicellular phytoplankton, bacteria, flagellates, and organic matter [44; 45; 46; 47], and will likely compete for these food items with other small zooplankton, such as other larvae and rotifers. They are also likely to be preyed on by larger zooplankton, such as copepods [38]. Relationships between bivalve larvae and these other taxa could therefore be expected. However, multiple correlations of bivalve and gastropod larval abundances with all possible prey species, competitors, and predators revealed no significant correlations with the abundances of any of these species. The possibility of indirect effects should, however, not be ruled out. It is difficult in studies such as these to capture the fine scale temporal changes with a limited  
Figure 10: Conceptual diagram of direct and indirect effects of increased temperature and HBCDD addition in model ecosystems. number of \"snapshot\" sampling times. Processes or events that have happened in between sampling points may well be the most important in determining the effects that are subsequently measured. Most of the common gastropod species present in shallow Baltic boys (e.g., _Hydroobia centrosa_, _H. neglecta_, _Potamopyrgus antipodarum_, _Theodoxus fluviatilis_, _Bithnyia tentaculata_, _Radix_ spp.) either produce live young, or lay eggs that hatch into juveniles. The only common species that has a larval stage, hatching from eggs laid on the seabed, is _Hydroobia ulvae_[48], which was most probably the species introduced to the cosms (\"Hydrobiidae\") at the start of the experiment. Adult Hydrobiidae increased in number from 10 at the start to c. 25 at 16 and >35 (not quantified) at t13, with no significant differences between treatments at t6. However, the larval abundance showed a clear treatment difference, with warm treatments having higher numbers at t6 and ambient treatments having higher numbers at t13. Given that the time to hatching takes at least 5 days, it seems probable that the larvae seen in this experiment hatched from eggs already present in the sediment at the start, a process that was stimulated (or occurred earlier) in the warm treatments. Fish and Fish [49] showed that _H. ulvae_ from a Welsh estuary had a clear optimum temperature with respect to embryonic development and survival as well as the time taken for eggs to hatch, as is the case for most invertebrates [3]. _H.