The Role of Light, Predation and Flow in Structuring Marine Subtidal Communities

University Of Massachusetts Boston, Dorchester MA

Investigators

Abstract

Marine invertebrate communities on vertical or undercut rock walls often differ dramatically from those on horizontal surfaces. Vertical walls typically harbor high-diversity assemblages of sessile suspension-feeding invertebrates, while horizontal surfaces tend to be dominated by macroalgae. Commonly invoked explanations for this pattern include: 1) Shading, which may slow algal growth and therefore favor invertebrate dominance. 2) Lack of disturbance, particularly predation, on vertical surfaces. 3) Lack of sedimentation on walls, thus freeing suspension feeders from clogged filtration systems. 4) Increased water flow on walls, which may enhance larval recruitment and food supply to suspension feeders. However, the actual roles of these different processes in shaping this ubiquitous pattern have yet to be tested experimentally. This project will investigate the relative importance of shading, predation, and flow in creating differences in community structure and composition between vertical and horizontal surfaces. The main predictions to be tested are: 1) Rich invertebrate animal communities develop on walls due to reduced light levels. This hypothesis will be tested by using neutral-density tinted acrylic to shade horizontal rock surfaces and using mirrors to augment light on vertical surfaces. 2) Predation limits the development of invertebrate communities on horizontal surfaces. This hypothesis will be tested via two separate treatments: removal of sea urchins (major invertebrate predators) from experimental plots, and caging to exclude all predators. 3) Increased water flow facilitates development of invertebrate communities. Flow over horizontal surfaces will be augmented via acrylic baffles mounted above the substratum. Experimental treatments will be executed in all possible combinations to detect the relative importance of each of the processes examined, as well as interactions among processes. All work will be conducted at a field site in Nahant, Massachusetts, in the southern Gulf of Maine. Sessile suspension feeding invertebrates can be very long-lived, and may be keystone habitat modifiers for a diverse marine community. The forces maintaining subtidal rock wall communities have yet to be rigorously examined. This study will provide important insight into the processes that regulate subtidal community structure, and enable prediction of the impact of anthropogenic changes on these processes. The generality of the pattern to be investigated suggests that results may apply to subtidal habitats globally.