Ecology of relic marine assemblages in unique marine-groundwater influenced lake habitats

A rare estuarine habitat occurs where parts of inland lakes are fed by marine groundwater. Stable influx of seawater can provide salinities suitable for littoral marine species, which often have persisted in such lakes since... [ view full abstract ]

A rare estuarine habitat occurs where parts of inland lakes are fed by marine groundwater. Stable influx of seawater can provide salinities suitable for littoral marine species, which often have persisted in such lakes since times when they were directly open to the sea. These relic populations live in conditions of much greater physical stability relative to almost all other habitats where they occur normally, allowing observation of ecological structure in uniquely sheltered conditions. For example, the interface between benthic rocks and the underlying sediment has previously been suggested as an important habitat feature, but testing benthic species associations to this feature is difficult when rocks and sediment are regularly disturbed. In this study of a marine lake in South Australia, it was measured on the extremely stable boulders how species are associated with the boulder/sediment interface at fine-scales. Also tested was a previously described correlation of Peracaridan (Crustacea) densities with different sized boulders, possibly occurring in exposed habitats due to variable disturbance related to boulder size/stability.

The results showed that a common anemone, Isanemonia australis, was strongly associated with the boulder/sediment interface, highlighting the importance of this habitat feature. Similar to previous findings, the correlation between densities of Peracaridans (in this case the isopod Zuzara venosa) and boulder size was significant. As this correlation occurred in a sheltered lake, this suggests that disturbance is unlikely a cause for this pattern that has been described widely for other similar species. Physical aspects of boulders themselves are likely responsible. Overall, this study highlights the potential for novel ecological tests using this unique estuarine habitat.