Eelgrass Bed Structure, Leaf Nutrient, and Leaf Isotope Responses to Natural and Anthropogenic Gradients in Estuaries of the Southern Gulf of St. Lawrence, Canada

Abstract

Seagrasses are declining globally; understanding and monitoring these trends forms an important basis for management practices. This study examined eelgrass (Zostera marina) structural and isotopic indicators as a means of characterizing stress from the impacts of agricultural land use in four estuaries in the Southern Gulf of St. Lawrence, Canada. These estuaries ranged from 5 to 62% agricultural land use in their respective watersheds. Anthropogenic stressors such as increased nutrient loading and decreased light penetration (from sedimentation and phytoplankton) and natural stressors such as low salinity and temperature were measured concurrently with plant structural indicators, e.g., aboveground biomass, belowground biomass, canopy height, and root-to-shoot ratio, across the spatial range of eelgrass presence in each estuary. Plant isotopic indicators included leaf tissue concentrations of C, N, S, and stable isotopes thereof and were measured at each station in eelgrass leaf tissue. Eelgrass plant indicators varied significantly across the gradients within each estuary as well as throughout the growing season, but due to the very high within-estuary variability indicators did not vary significantly between estuaries. Eelgrass leaf nutrients and stable isotopes also varied significantly between stations within estuaries though in this case, among-estuary differences were detected. A distance-based linear model was employed to examine the ability of estuarine characteristics and plant isotopic and elemental variables to predict the plant structural indicators. Results showed that factors related to both salinity and nitrogen loading influenced the biomass and structure of this plant but in different ways. Belowground biomass most correlated to salinity, and aboveground biomass most related to nitrogen loading with a strong relationship with light attenuation. Overall, the high level of within-estuary variability, and the strong influence of salinity, suggests that comparison of plant structural indicators between estuaries may have limited utility as a monitoring tool; eelgrass coverage throughout an estuary may better reflect the influence of land use at a regional scale.