Impacts of boat mooring disturbance on productivity and respiration in rhodolith beds from Catalina Island, USA

Aggregations of rhodoliths, habitat-forming, free-living coralline red algae, form beds throughout the world's oceans. On Santa Catalina Island, California, USA, rhodolith beds occur in protected coves where dense networks of moorings support recreational boating activities. The chains and spreader lines associated with these moorings chronically disturb the benthos, crushing the rhodoliths and reducing biodiversity of rhodolith-associated communities. Here, we examine how mooring disturbance affects rhodolith photosynthesis and respiration and characterize rhodolith-associated invertebrate respiration to better understand how this disturbance affects productivity by the ecosystem. To do this, we used a respiration chamber in the laboratory to measure the amount of oxygen produced and/or consumed by undisturbed (intact) rhodoliths, mooring-disturbed (crushed) rhodolith fragments, and laboratory-crushed rhodoliths, and the amount of oxygen consumed by the dominant rhodolith-associated invertebrate taxa. Our results indicate that rhodolith maximum net productivity is significantly reduced and rhodolith respiration is significantly increased by mooring disturbance in the field, but that crushing of the rhodolith thalli alone does not result in immediate changes to either of these measures. Rather, it appears that chronic crushing of rhodolith thalli, which results in their mortality and rhodolith habitat degradation, is required to elicit these metabolic changes. In addition, we observed variation in respiration rates among the 5 most commonly observed invertebrate taxa within the Catalina Island rhodolith beds, and scaling these respiration rates by each species' abundance in the rhodolith beds and in adjacent mooring-disturbed (degraded) habitats indicated that mooring disturbance results in a decrease in community respiration by approximately 2.61 mg O-2.m(-2).d(-1), with individual species contributing between 0.05 mg O-2.m(-2).d(-1) and 1.84 mg O-2.m(-2).d(-1) to this decrease. This study provides insight into the consequences of anthropogenic disturbance on productivity and respiration in these ecologically important habitats.