Environmental and biotic habitat attributes affect rocky intertidal community variability in glacially influenced estuaries

Abstract

Climate change is increasing the rate of glacial recession in high latitude coastal environments. The associated increase in cold, sediment-laden freshwater into the nearshore may alter a wide array of water attributes, which will likely have ecosystem-wide impacts. One of these potential impacts is a change in typical patterns of ecological community variability, which can be an indicator of stress in response to an environmental impact. Fluctuation in community composition over multiple temporal scales is natural and can be affected by environmental conditions, as well as by the presence of habitat forming, spatially dominant organisms. The goal of this study was to determine how variation in environmental attributes and spatially dominant species contributes to the seasonal variability of rocky intertidal community assemblages in glacially influenced estuaries. The environmental attributes of interest were both dynamic in nature (water temperature, salinity, dissolved oxygen, turbidity, and pCO₂₎, and static (substrate type, wave exposure, beach slope, and distance to freshwater). Variation in the proportions of three spatially dominant organisms that are common in many intertidal systems, Mytilus trossulus (mussels), Balanus spp. (barnacles), and Fucus spp. (rockweed) were examined as biotic habitat attributes. It was hypothesized that 1) dynamic environmental attributes would be more strongly correlated to community variability patterns than static attributes, and 2) higher proportions of spatially dominant species would correspond to lower levels of community variability. To document seasonal community variability, as well as proportions of spatially dominant species, percent cover and biomass data were collected along with environmental data from 2019 to 2022. Barnacle cover, rockweed cover, and substrate characteristics (cover of gravel and total cover of bare rock) were significantly correlated to community variability levels based on percent cover data. Community variability levels based on biomass data were significantly correlated to mussel cover, rockweed biomass, substrate aspects (cover of gravel and mud), distance to a freshwater source, and variation in dissolved oxygen levels. Almost all relationships between community variability and each of these attributes were negative, where a greater level of an environmental attribute was associated with lower seasonal variability in community composition. Distance to freshwater and dissolved oxygen variation had a positive relationship to community variability, meaning that greater distance to a freshwater source and variation in dissolved oxygen were associated with higher seasonal community variability. As several of these variability drivers may be affected by climate-related changes, the results of this study provide insights into how glacially influenced rocky intertidal communities may in turn be affected by the progression of climate change.