Temporal variation in intertidal habitat use by nekton at seasonal and diel scales

Abstract

Structure provided by temperate seagrasses is expected to reduce encounter rates with predators and therefore benefit small nekton most in summer, when predation is intense and seagrass reaches peak biomass, and in the day relative to night, when darkness limits visually-oriented predators regardless of habitat. Based on seines in eelgrass (Zostera marina), unvegetated habitat, and edges in Willapa Bay, Washington, USA, nekton abundances varied across habitats and on both diel and seasonal time scales, yet only a few time-by-habitat interactions were observed, in which habitat distinctions were most pronounced at certain times. One explanation is that four of the six most abundant species disproportionately occupied unvegetated habitat or were habitat generalists, but our expectations were based on eelgrass-associated taxa. Multivariate community structure responded separately to season, habitat, and diel period, in order of importance. Total abundance showed a significant season-by-habitat interaction. A summer peak in vegetated habitats was largely driven by shiner perch and sticklebacks, two eelgrass-associated fishes, while unvegetated habitat showed year-round uniform abundances due to taxa like English sole and sand shrimp with winter and spring peaks, and no strong habitat associations or association with unvegetated habitat, respectively. In a single diel-by-habitat interaction, shiner perch were eelgrass-associated during the day but not at night. No evidence emerged of differences in body size across habitats for any species measured, but many taxa grew seasonally, likely as cohorts migrating out of the estuary. Seasonality was thus the strongest signal governing patterns of community structure, abundance, and body size across time and space, while habitat structure and diel period were less important. Our results are largely consistent with the other studies showing the primacy of seasonality in structuring estuarine nekton communities, but also show that this pattern is highly dependent on the seasonal recruitment patterns and habitat associations of abundant nekton.