Nonequilibrium Dynamics

If more detailed descriptions and predictions are required (e.g., the diversity and abundance of individual species and other specific ecosystem services and assets), then the predictive ability is less. One of the reasons for this is the fact, alluded to above, that these are nonequilibrium systems which respond to individual events (storms and engineering works) over long time periods. The elimination and invasion of species may take decades and the responses of freshwater lagoons, for example, to salt incursions may also take decades. A particularly good example is Lake Wellington in the Gippsland Lakes system in Victoria, Australia. The entire system is slowly responding to the ingress of salt made possible by the opening of the lagoon system mouth (Lakes Entrance) in 1883. Lake Wellington, the lake farthest inland, remained fresh until after the 1967 drought when a combination of high N and P loads from agriculture, the extraction of water from the inflowing La Trobe River for power station cooling and irrigation, and the incursion of salt killed all the freshwater macrophytes in the Lake. In a few years the lake switched from its previous clear and macrophyte dominated state to being turbid and dominated by toxic algal blooms. It does not appear to be possible to switch it back.

The response of these lagoon systems to climate and other perturbations is nonlinear and complex because of the interactions between the major functional groups and because the timescales of response of the major groups differ strongly. Phytoplankton may respond to changes in loads and water residence times in a matter of days, whereas seagrasses take decades or longer to recover. By perturbing a simple coupled plankton-benthos model with storm events and 'spiked' N loads, Webster and Harris showed that the threshold load for the elimination of seagrasses could be altered considerably depending on the characteristics of the input loads. So the response of the system was a function of the overall load and the frequency and magnitude of events. Climate change and catchment development both alter the overall C, N, and P load to lagoons as well as the characteristics of that load, so that ecological responses by lagoons are highly complex and change over time depending on a variety of modifications and management actions. Consequently, lagoons are always responding to the last storm or intervention and the abundance of key species drifts to and fro over time as the entire plankton-sediment system responds.

The picture is made more complex by the evidence for strong trophic cascades in marine as well as freshwater systems. Coastal ecosystems are frequently over-fished; larger predators and grazers are removed by human hand. Removal of the 'charismatic megafauna' of coastal systems, together with beds of shellfish and other edible species, has changed the ecology of many lagoons and estuaries. Coastal ecosystems around the world have also been strongly modified by the removal of natural physical structures (mangroves and reefs) which confer resilience in the face of extreme events. We have removed both larger fish and benthic filter feeders from many systems compromising function and the ability to respond to changes in catchment loads. Overall there has been a consistent simplification of both physical and ecosystem structures (removal of reefs and macrobiota, simplification of food chains, etc.) and a trend toward more eutrophic (nutrient rich) and simplified systems dominated by microbiota, especially algae and bacteria. We know less about the response of ecosystems to changes in the 'top down' trophic structure than we do about the responses to 'bottom up' catchment drivers; nevertheless, there is good evidence for similar nonlinearities and state switches in response. A nonequilibrium view of coastal lagoons changes the way we look at them. Overall there is a need to pay attention to the 'precariousness' of these systems and manage them adaptively for resilience and response to natural and anthropogenic impacts. Despite being over-fished and highly modified, there is still a need for the ecosystem services they produce.

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