A mass-balanced food web model for a kelp forest ecosystem near its southern distributional limit in the northern hemisphere

Coastal ecosystems are influenced by a suite of drivers and interactions, resulting in complex dynamics not captured by single species, single driver approaches. Kelp forest ecosystems of the California Current region are subject to extreme environmental variability as well as a suite of fishing pressures which remove organisms throughout the food web. Here, we present a food-web model to assess ecosystem-wide effects of different fishing strategies which can also be used to estimate the effect of warming, acidification and hypoxia on the structure and function of a highly productive temperate ecosystems. We built a mass-balanced model of a kelp forest ecosystem near the southern limit of distribution in the northern hemisphere (Isla Natividad, Mexico). The model is informed by extensive ecological monitoring of fish, benthic invertebrates, and macroalgae conducted annually from 2006 to 2016 at 5 sites around the island. The model includes 40 functional groups (FG) defined on the basis of commercial interest, ecosystem function and feeding guild, including birds, marine mammals, fish (13 FG), commercial species (7 FG), macroinvertebrates (8 FG), zooplankton, the giant kelp Macrocystis pyrifera, six macroalgae, phytoplankton and detritus. The estimated throughput was 5477.6 t·km−2·year−1 (26% consumption, 27% exports, 24% respiration and 23.7% detritus). The sum of all production was 2727.9 t·km2·year−1, and the total primary production/total respiration ratio was 2.4. This model, based on a detailed, multi-year monitoring program in the kelp forests, provides a valuable tool for exploring drivers of change in these vulnerable marine ecosystems and fisheries.