Seasonal shifts dominate mesozooplankton community structure and size-based functional variation in a coastal offshore photovoltaic facility

This study assessed seasonal and spatial variation in mesozooplankton communities within a nearshore fixed offshore photovoltaic (PV) facility and adjacent non-PV waters in Laizhou Bay, China. Mesozooplankton samples were collected in January and April 2026 and analyzed using ZooScan imaging to quantify abundance, biovolume, community composition, and size- and trophic-related indicators. Mesozooplankton abundance and biovolume increased markedly from January to April, from 175.3 ± 483.4 to 3298.6 ± 2973.5 ind./m 3 and from 39.7 ± 86.2 to 643.0 ± 573.6 mm 3 /m 3 , respectively. Community composition differed significantly between months, and the seasonal difference remained significant after excluding Noctiluca . The normalized biovolume size spectrum (NBSS) intercept also increased significantly from January to April. Within each month, total mesozooplankton abundance, community composition, and most size- and trophic-related indicators did not differ significantly between the photovoltaic area (PA) and adjacent non-photovoltaic area (NPA). Among the major mesozooplankton groups, copepods showed the clearest taxon-specific spatial patterns. Copepod biovolume was significantly lower in the PA than in the NPA in April, whereas in January, copepod size composition showed significantly greater within-area dispersion in the PA for both abundance and biovolume. These results indicate that seasonal variation was substantially stronger than PA–NPA spatial differentiation during the survey period, while evidence for a consistent community-wide PV-associated response was limited. These findings provide an initial field-based baseline for distinguishing natural seasonal variability from potential PV-associated effects and support long-term ecological monitoring of offshore PV facilities.