Environmental influence on size frequency distributions of the Pacific blue mussel (Mytilus trossulus) in two glacially inlfuenced estuaries

Abstract

The Pacific blue mussel (Mytilus trossulus) is a foundation species in high-latitude intertidal and estuarine systems that can create complex habitats, provide sediment stability, serve as food for top predators, and act as connectors between the water column and the benthos. M. trossulus also makes an ideal model species to assess biological responses to environmental variability, as its size frequency distributions can be influenced by the environment in which it lives. Size frequency distributions can provide valuable information about ecological systems that are experiencing environmental change (e.g., increased global temperatures). M. trossulus populations in high latitude estuaries receive freshwater runoff from snow and glacial-fed rivers or can be under oceanic influence. These hydrographic conditions work together with local static environmental variables, such as substrate, fetch (potential for wave action), beach slope, distance to freshwater, and percent glaciation (glacial discharge) to influence recruitment, growth, and mortality of mussels. In 2019 and 2020, M. trossulus was collected from 15 intertidal sites in two Gulf of Alaska ecoregions with varying hydrographic conditions to determine if and how mussel size frequencies change over spatial and hydrographic scales, and whether any static environmental characteristics correlated with this variability. This study demonstrated that M. trossulus size frequencies were most comparable at sites with similar hydrographic conditions and grouped according to the ecoregion and year of collection. M. trossulus recruits (0-2 mm) were mostly seen at sites with higher fetch, while large mussels (> 20 mm) were mostly seen at more protected sites (low fetch) and in areas with more freshwater influence. Hydrographic conditions explained approximately 43% of the variation in M. trossulus size frequencies for both years, which was three times more than the variation explained by ecoregion and four times more than collection year. Fetch and distance to a freshwater source explained most of the variation in mussel size frequencies for both years, while substrate type was also important in 2019, and percent glaciation in 2020. M. trossulus recruitment was significantly different between 2019 and 2020, possibly resulting in the different static variable correlates between the two years. This study suggests that hydrographic conditions play an important role in structuring M. trossulus size frequencies, and that these differences also depended on environmental conditions.