Collaborative Research: Kelp forest hydrodynamics: observations of drag and cross-shore exchange on the inner shelf

Stanford University, Stanford CA

Investigators

Abstract

Kelp forests are common to many mid-latitude coasts, and they are among the most valuable inner shelf habitats for fisheries, recreation, and possibly biological carbon fixation. This project seeks to quantify the wave-dependent drag forces and associated hydrodynamic mechanisms by which giant kelp mediates flow conditions in inner shelf habitats. The frictional drag generated by individual kelp plants and whole kelp forests strongly influence circulation and transport in coastal habitats. However, at present it is not possible to incorporate these effects into coastal circulation models. High-resolution field measurements will be used to develop parameterizations of drag on a whole kelp forest in the presence of coastal currents, surface waves, and internal waves, that will be suitable for use in inner shelf circulation models. The analysis will seek to quantify the magnitude by which kelp forests affect cross-shore exchange on the inner shelf and to determine the mechanisms that mitigate cross-shore exchange in the presence of kelp so that the findings may be generalized to any kelp forest environment. This is relevant to understanding coastal environmental flows and their interactions with a range of types of aquatic vegetation in both natural systems and for large-scale aquaculture and ecosystem management. Understanding their hydrodynamics is thus important to management of coastal waters, as well as to designing natural restoration areas and potential aquaculture systems. Given the rapid development in high resolution circulation models as decision-analysis tools for coastal zone management, including the potentially large hydrodynamic effects of kelp forests can be an essential prerequisite to producing accurate predictions of inner shelf flows. Thus, one particular focus of the project will be to develop a new model for kelp drag in terms of mean currents and wave velocities incorporating simple measures of kelp configuration and biomass that can be used in coastal circulation models. Given the fundamental roles hydrodynamics play in shaping kelp forest ecology through its effects on biogeochemistry and on the transport of larvae, advancing our understanding of kelp forest hydrodynamics will be of use to a wide range of researchers and resource agencies. Moreover, given the large body of ongoing work focused on the Pt. Loma kelp forest, as well as its fundamental importance to the California coastal ocean due to its large size, the results of this study should be of particular use to local scientists and managers. Accordingly, the data collected will also be archived on SCOOS servers as well as on the NODC database. The project will also support graduate education, post-doctoral professional development, and public outreach. The central theme of the project will be to quantify depth-dependent drag associated with flow imposed on flexible vegetative structures – kelp plants in currents and waves – where the movement of the kelp is both influenced by, and in turn mediates, the overall hydrodynamic conditions. Thus, the research will delineate mechanisms of the complex feedbacks between environmental flows and aquatic vegetation that can span the entire water column. While past studies in the field and in the lab have examined elements of this interaction, e.g. changes in mean flows or wave velocity fields by kelp, this study will provide a holistic view of kelp forest hydrodynamics particularly including motion of the kelp. Whereas, in previous field studies kelp drag has always been estimated in ad hoc ways, here it will be measured directly. Quantitative analysis of field observations will test three key hypotheses: (1) Movement of the plants in the presence of waves and sheltering effects that develop when kelp-plant density is sufficiently high are O(1) determinants of mean drag. This hypothesis implies that drag of a whole kelp forest is not a simple linear function of the number of individual kelp plants, but depends on interactions between scale and spacing of individual drag elements and their movement in imposed flows. (2) Wave-current interactions and frequency dependent radiation stress alter the flow field in and around kelp forests. These flow effects along with damping of high frequency internal waves and internal tides and increased diurnal thermal stratification imply that (3a) the presence of a kelp forest can locally enhance cross-shore exchange between the kelp forest and the offshore ocean environment and (3b) a kelp forest acts as a partial barrier that reduces transport between the offshore region and the very near shore region inshore of the kelp. The latter hypothesis predicts the presence of kelp forests can have important consequences for altering circulation and residence time near shore. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.