Plankton supports the majority of fish biomass on temperate rocky reefs

Coastal temperate rocky reefs are economically valuable and highly diverse, yet the trophodynamics of these productive systems are understudied. Quantifying the trophic linkages that support fish assemblages on these reefs is valuable for understanding how these assemblages may change due to changes in benthic and pelagic primary production. The goal of this study was to quantify the relative importance of primary sources of nutrition (here, macroalgae, phytoplankton, zooplankton, and detritus) for supporting fish assemblages on shallow rocky reefs. We constructed a general food web that traces the food sources supporting fish biomass on shallow temperate reefs near Sydney, Australia, using species composition data from 17 reefs based on field sampling and the Reef Life Survey (reeflifesurvey.com), and combined this with dietary information sourced from field collections and published literature. Planktivorous fish typically dominated reef fish assemblages, comprising an average of 41% of the total fish biomass (ranging from 12 to 71% among sites). Our food web analysis showed that, on average, 56% (±5% s.e.) of the total fish biomass was ultimately supported by phytoplankton (and 53% from zooplankton), in comparison to 31% (±4%) supported by macroalgae and 12% (±2%) by detritus. This result highlights the dominance of zooplanktivorous fish on temperate reefs and also their importance as prey for piscivores. Our findings demonstrate the importance of understanding the coastal dynamics of plankton and planktivory for predicting the response of temperate reefs and their fish assemblages to a changing climate.