The influence of multiple environmental stressors on susceptibility to parasites: An experimental determination with oysters

Abstract

A large-scale field experiment was used to test whether exposure to a suite of potential environmental factors (flow speed, temperature, salinity, and low dissolved oxygen) influences the level of parasitic infection of the oyster Crassostrea virginica. The parasite was the protozoan Perkinsus marinus, which has decimated populations of oysters in estuaries of North America. The environmental factors were considered stressors because they influence the physiological condition of either the host or parasite. Between December 1994 and July 1995, flow speed, temperature, salinity, dissolved oxygen concentration (DO), Perkinsus infection, and mortality of oysters were monitored across 24 experimental oyster reefs in the Neuse River estuary, North Carolina. Eight reef treatments were created consisting of an orthogonal combination of three factors: water depth (3 m vs. 6 m deep), reef height (2 m vs. 1 m tall), and position on reef (base vs. crest). Principal component analysis revealed that there was clear separation of environmental factors among reefs and that a majority of the variation (96.2%) among treatments could be explained by two principal component axes (PCs): one (24.3% of variation explained) was formed by flow and the other (71.9% of variation explained) by temperature, salinity, and DO. Oysters with the highest proportion of individuals infected (prevalence), highest intensity of infection, and highest mortality were located at the base of reefs, where flow speeds and food quality were lowest and sedimentation rates highest. However, there was no significant effect of hydrographic conditions on Perkinsus infection or mortality of oysters, despite large differences in mean salinity, DO, and exposure to hypoxia-anoxia. Temperature did not vary among treatments. Correlation of disease responses (infection prevalence and intensity, and mortality) with the first two PCs showed that these response variables were significantly (and negatively) correlated with flow only. Oysters in low flow were hypothesized to have the greatest susceptibility to Perkinsus infection because of their poor physiological condition. The restoration of oyster reefs increases reef height and thus flow speeds (by elevating oysters higher above the boundary and by actively influencing flow), thereby reducing the negative effects of disease by restoring reef morphology to its natural size.Interactions between parasites and their hosts can dictate host population dynamics (Burdon 1987), alter food webs

Variability in responses to nutrients and trace elements, and transmission of stressor effects through an estuarine food web

“…The extent of this specificity can be influenced both by their mode of action and by the level of exposure to which species are subjected. Even when considering direct effects on a single trophic level, susceptibility can vary among species (e.g., Magnuson et al 1989;Sanders and Riedel 1998;Diaz and Rosenberg 1995;Williamson et al 1999) and may be influenced by the presence or intensity of other stressors in the environment (e.g., Folt et al 1999;Lenihan et al 1999). A potential consequence of this variation is that functioning of the susceptible trophic level may depend on species composition at any particular point in space or time as well as on the intensity and duration of the system's exposure to the spectrum of stressors in the environment.…”

National Center for Ecological Analysis and Synthesis, Santa Barbara, California 93101 and Center for Limnology,University of Wisconsin, Madison, Wisconsin 53706

“…According to this definition, stressors cause either an increase or a decrease in a focal parameter; prefixes can be used to denote the direction of the effect (e.g., distress vs. eustress; Rapport et al 1981). Alternatively (and perhaps more intuitively), a stressor is any factor that has a negative effect on an ecological system (e.g., Odum et al 1979;Barrett 1981;Odum 1985;Howarth 1991); obvious examples include acidification (Schindler 1990;Frost et al 1999;Leavitt et al 1999) and disease (Lenihan et al 1999). By this definition, factors which have positive effects are ''subsidies,'' not stressors.…”

Bottom Habitat Mapping Using Towed Underwater Videography: Subtidal Oyster Reefs as an Example Application

“…In contrast, the Squamscott reef consisted largely of live oysters with many clusters extending several centimeters into the water column ( Figure 4). Vertical structure is a major characteristic of oyster reefs that contributes to their value as habitat for other organisms, as well as health of the reef itself (LENIHAN, 1999;LENIHAN et al, 1999;PETERSON and ESTES, 2001). This is an important reef characteristic that should be a part of monitoring programs for reef restoration projects (COEN and LUCKEN-BACH, 2000;LUCKENBACH et al, 2005).…”