Catriona L. Hurd

Release of dissolved organic carbon (DOC) by seaweed underpins the microbial food web and is crucial for the coastal ocean carbon cycle. However, we know relatively little of seasonal DOC release patterns in temperate regions of the southern hemisphere. Strong seasonal changes in inorganic nitrogen availability, irradiance, and temperature regulate the growth of seaweeds on temperate reefs and influence DOC release. We seasonally surveyed and sampled seaweed at Coal Point, Tasmania, over 1 year. Dominant species with or without carbon dioxide (CO 2 ) concentrating mechanisms (CCMs) were collected for laboratory experiments to determine seasonal rates of DOC release. During spring and summer, substantial DOC release (10.06–33.54 μmol C · g DW −1 · h −1 ) was observed for all species, between 3 and 27 times greater than during autumn and winter. Our results suggest that inorganic carbon (C i ) uptake strategy does not regulate DOC release. Seasonal patterns of DOC release were likely a result of photosynthetic overflow during periods of high gross photosynthesis indicated by variations in tissue C:N ratios. For each season, we calculated a reef‐scale net DOC release for seaweed at Coal Point of 7.84–12.9 g C · m −2 · d −1 in spring and summer, which was ~16 times greater than in autumn and winter (0.2–1.0 g C · m −2 · d −1 ). Phyllospora comosa , which dominated the biomass, contributed the most DOC to the coastal ocean, up to ~14 times more than Ecklonia radiata and the understory assemblage combined. Reef‐scale DOC release was driven by seasonal changes in seaweed physiology rather than seaweed biomass.