In situ community contributions to benthic biogeochemistry in temperate rocky reefs

The benthic zone within kelp forests is critical habitat to many organisms and has reduced water motion from kelps’ structural biomass and friction with the seafloor. Such reduced flow may enable benthic organisms to contribute to local water chemistry. Community composition in the benthic understory is variable and community members likely contribute differently to biogeochemical patterns. With shifts to alternative states such as urchin barrens, there may be biogeochemical consequences with the loss of producers. Here we assess in situ community contributions to biogeochemical variability in the benthic zone of temperate rocky reefs. Dissolved oxygen (DO) and temperature were continuously measured along with discrete measurements of pH directly in and 1 meter above bottom (MAB) in three benthic community types within coastal Monterey Bay: a tall dense understory algal community within a kelp forest, a patchy understory algal community within a kelp forest, and an urchin barren lacking kelp and understory algae. Sensors were paired with dissolution blocks to assess relative water motion. Universal point contacts and understory height measurements were used to assess community composition and structure, which differed significantly between community types. Differences in DO between communities were strongest at the bottom compared to 1 MAB with temporal variation likely driven by physical processes as properties of offshore water masses changed depending on upwelling strength and the presence of internal bores. Patchy understory communities showed similar DO across depths but differences in relative water movement between bottom and 1 MAB with bottom having the lowest flow. In the tall understory and urchin barren, relative water movement was similar across depths and DO was higher 1 MAB compared to bottom. This study shows that benthic communities contribute to in situ biogeochemical and physical environment, shaping distinct microhabitats of benthic organisms.