Estuaries are among the most human-dominated of aquatic ecosystems. A recent challenge comes from the global expansion of mariculture, which may improve sustainable yields of marine resources, but can transform habitat in ways similar to terrestrial agriculture. Key species of management concern are seagrasses, which are protected by “no net loss” provisions in the absence of much information on their resistance or resilience to perturbations from mariculture practices.
Our research efforts in Washington state, from 2001-2008, have addressed five major conceptual ideas about responses by the native eelgrass, Zostera marina, to bivalve shellfish mariculture: 1) the capacity for resistance and resilience across multiple morphotypes of eelgrass; 2) space competition from bivalves; 3) facilitation through benthic-pelagic coupling that alleviates light or nutrient limitation; 4) ecosystem engineering by eelgrass; and 5) factors governing transitions to alternative stable states. These research questions have been pursued through crossed experimental designs in which disturbance to eelgrass and addition of cultured bivalves are manipulated independently, with results then compared to large-scale observational studies across farmed and unfarmed areas.
Overall, we show that competitive effects of bivalves have trumped facilitation, despite evidence to the contrary collected in other parts of the world involving different species pairs. Resilience has trumped resistance to disturbance, with small gaps generally recovering in less time than a typical shellfish crop cycle. An important life history event enabling recovery of small gaps is asexual reproduction (rhizome branching), which occurs seasonally in all studied populations, but effective recruitment from seed is much less common, and this could limit resilience to larger-scale removal. Finally, the role of eelgrass as an ecosystem engineer has been strongly supported in terms of physical changes to water flow and sediment, with the engineering roles of bivalves much more variable across taxa. Ecosystem engineering is one mechanism by which alternative states may be maintained, but aquaculture alone does not appear to cause a permanent shift from an eelgrass state. At the same time, evidence is accumulating that introduced bivalve reefs and dynamic sediments may have positive feedbacks that make it difficult for eelgrass to recover after loss. Aquaculture activities that fragment reefs and stabilize sediment could therefore foster the appearance of eelgrass. Such studies of aquatic agro-ecosystems lag far behind their terrestrial counterparts but are essential scientific underpinnings to management for sustainable ecosystem services in estuaries.