Interactive roles of mesograzers and current flow in survival of kelps

Abstract

The relationship between hydrodynamic energy and biological processes is examined for a kelp-dominated marine community in the San Juan Archipelago. Populations of a common and widely distributed kelp, Nereocystis luetkeana, were established and rates and causes of mortality followed at 7 sites differing greatly in tidally driven current velocities and wave exposure (measured by permanently deployed instrument packages). Mortality of N. luetkeana was not related to storm energy, but exhibited a significant non-linear relationship with tidal current energy such that mortality rates were highest at sites exhibiting protracted periods of calm punctuated by episodes of strong currents. The role of mesograzers (primarily the gastropod Lacuna vincta) on survivorship in these kelp populations was evaluated in the field and in laboratory flume experiments. The relationship between grazer damage and stipe breaking force was investigated by measuring the tensile forces required to break experimentally damaged stipes. Although undamaged stipes can easily withstand the tensile forces imposed by even the strongest current and wave exposures, a very small amount of damage will have highly significant negative effects on breaking strength. While apparently responsible for a significant portion of N. luetkeana mortality in low and variable energy environments, L. vincta is unable to persist on kelp stipes in high-energy environments and its role there is trivial. The relationship between L. vincta grazing and hydrodynamic energy is however nonlinear, because water movement has opposite effects on grazer foraging behavior and the drag forces imposed on kelps, and this results in a complex relationship between hydrodynamic energy and kelp survival. We suggest a conceptual model for relating kelp survival to grazing intensity in hydrodynamically variable environments. The model leads to the prediction, exhibited in our field results, that the probability of plant mortality may be maximized in regions of intermediate flow energy.