Foundation species consist of plants (e.g. seagrasses), algae (e.g. kelps) or animals (e.g. oysters, corals) that create habitat for a wide diversity of fauna. While foundation species support resident fauna, these residents also can impact foundation species, either positively by facilitation or negatively through competition or predation. Human-induced changes such as species introductions, warming, and eutrophication can alter these interactions. Using eelgrass (Zostera marina) as a study species, I explored how species introductions, warming, eutrophication, and habitat disturbance affected interactions between eelgrass and bivalves, which are common residents of eelgrass habitat. My first chapter focused on the effects of a non-native seagrass species on Manila clams in Puget Sound, WA. Aquaculture of Manila clams is an important industry in Washington State. Non-native Zostera japonica has colonized mudflats in Puget Sound often used by Manila clam growers, which has created concerns about their effects on Manila clams and the industry they support. I analyzed a data set from a field experiment conducted by personnel from NOAA that tested for effects of habitat type (native eelgrass Zostera marina, non-native eelgrass Zostera japonica, and unvegetated mudflats) on Manila clam growth, survival, reproductive status, and condition. I found no evidence that Zostera japonica had negative effects on Manila clams; rather, differences in clam growth, survival, reproductive status, and condition were primarily driven by site-level differences such as temperature. My results do not indicate that Manila clam aquaculture will be negatively affected by Zostera japonica expansion, although my research did not address whether the process of Manila clam harvesting may be affected by the transformation of mudflats into vegetated habitat by Z. japonica.
In Chapter 2 I focused on the effects of temperature and nutrients on clam-eelgrass interactions. Bivalves and eelgrass coexist in a variety of environmental conditions: while some studies find that bivalves have positive effects on eelgrass, by (e.g.) improving water clarity or depositing nutrients via pseudofeces, other studies have found negative effects due to competition for space or altered sediment chemistry via biodeposition. Eelgrass also coexists with a variety of crustacean and gastropod mesograzers, which benefit eelgrass by consuming epiphytic algae growing on its surface. I postulated that eelgrass-bivalve interactions may vary with environmental context; specifically, bivalves might be more likely to facilitate eelgrass under stressful conditions. I also postulated that mesograzer effects on eelgrass would be consistently positive across different conditions. My laboratory experiments indicated that clam effects on eelgrass growth or survival did not vary with temperature or nutrient treatments. However, I found that gastropod mesograzers may confer resilience to temperature stress for eelgrass, indicating that these epifauna may be important in maintaining eelgrass as temperatures rise due to climate change.
In chapter 3 I tested for joint effects of two types of disturbance on eelgrass infauna: habitat alteration by non-native Asian mussels, and small-scale eelgrass removal that commonly occurs from human use of estuaries. In shallow estuaries that support eelgrass, small-scale eelgrass removal via anchor scarring, propellor scarring and other physical mechanisms commonly co-occurs with the proliferation of space-occupying invasive species such as Asian mussels. I conducted a field experiment testing the effects of eelgrass removal and addition of Asian mussels on eelgrass infauna, and hypothesized that when eelgrass was removed, Asian mussels would compensate for eelgrass loss by providing habitat for infauna, thereby sustaining a high diversity and abundance of organisms. I found limited effects of eelgrass disturbance on infauna, and overall higher infaunal abundance when Asian mussels were present. However, I did not find evidence that Asian mussels could compensate for eelgrass loss. Infauna may not have responded to disturbances and Asian mussels due to the regular occurrence of disturbances and high densities of mussels already present in heavily used estuaries like those in Southern California.
Overall, results of my three studies indicate that despite potentially strong effects of environmental factors such as temperature on eelgrass fauna, the native species that I studied exhibited resilience to species invasions and habitat alteration. The ecological roles of eelgrass fauna should be the focus of future research and considered in seagrass conservation and restoration efforts.
