Influence of Kelp Forest Biomass on Nearshore Currents

Abstract

As part of a project focused on the coastal fisheries of Isla Natividad, an island on the Pacific coast of Baja California, Mexico, we conducted a 2‐1/2 year study of flows at two sites within the island's kelp forests. At one site (Punta Prieta), currents are tidal, whereas at the other site (Morro Prieto), currents are weaker and may be more strongly influenced by wind forcing. Satellite estimates of the biomass of the giant kelp (Macrocystis pyrifera) for this period varied between 0 (no kelp) and 3 kg/m2 (dense kelp forest), including a period in which kelp entirely was absent as a result of the 2014–2015 “Warm Blob” in the Eastern Pacific. During this natural “deforestation experiment”, alongshore velocities at both sites when kelp was present were substantially weaker than when kelp was absent, with low‐frequency alongshore currents attenuated more than higher frequency ones, behavior that was the same at both sites despite differences in forcing. The attenuation of cross‐shore flows by kelp was less than alongshore flows; thus, residence times for water inside the kelp forest, which are primarily determined by cross‐shore velocities, were only weakly affected by the presence or absence of kelp. The flow changes we observed in response to changes in kelp density are important to the biogeochemical functioning of the kelp forest in that slower flows imply longer residence times, and, are also ecologically relevant in that reduced tidal excursions may lead to more localized recruitment of planktonic larvae.

Surface waves and currents in aquatic vegetation

“…Based on field experiments, Utter & Denny (1996) estimate a value of for macroalgae, and Monismith et al. (2022 a , b ) analyse other aspects for kelp-forest drag. Luhar & Nepf (2011) discuss how plant compliance under flow alters its configuration, hence .…”

Capturing accurate kelp canopy extent: integrating tides, currents, and species-level morphology in kelp remote sensing

“…Together, these examples demonstrate that both NDVI and NDRE may accurately classify dense kelp at the center of a bed where VI n values are consistently higher, while kelp at the bed edges may be more accurately classified using NDRE due to the improved detectability of submerged kelp. We were unable to measure why kelp was more submerged at the bed periphery, but we hypothesize that it may be due to slightly higher current speeds at the bed-edge submerging more canopy than at the center of the bed where currents are reduced (Monismith et al, 2022). This level of analysis is important for when deriving kelp bed biomass based on NDVI (K. Cavanaugh et al, 2011) because our results suggest that dense kelp at the periphery of a bed will have a lower NIR R 0+ if submerged.…”