Ecological effects of overwater structures on subtidal kelp, northern Puget Sound, Washington

Subtidal kelp, over 20 species in total, are abundant in the Puget Sound but no data exists on how they are impacted by the multitude of overwater structures found there. There are currently over 9,000 overwater structures creating 9 km2 of shade in the Sound. This research quantifies various overwater structures’ impacts on the productivity and distribution of subtidal kelp beds. Three sets of floating docks ranging in size from 35 to 66 m2, located at Deception Pass and Camano Island State Parks, were sampled twice during the summer of 2017. Georeferenced underwater video surveys were conducted by paddling a floating platform with a depth adjustable camera/laser array to record the presence of subtidal kelp, which was primarily sugar kelp (Saccharina latissimi), although all species were surveyed. Parallel transects out to eight meters from each dock and in each control, spaced two meters apart, were followed using an on-board GPS. The transects and underwater video footage were correlated, using their time stamps, to create one-meter grid cells which were encoded and mapped to show presence of subtidal kelp. Wet biomass and morphometric measurements were taken from kelp collected from thirty samples along the transects at each site. Data was also collected for related environmental conditions at each site, including light extinction, depth and substrate. Light extinction coefficients were calculated using an array of eleven photosynthetically active radiation sensors deployed at various depths and distances from each dock, as well as within each paired control site. Substrate samples taken with a Peterson dredge, along each transect, were analyzed for organic content and particle size distributions. Proportional coverage and densities of subtidal kelp were statistically compared for significant differences between the docks and their paired control sites and were correlated with related environmental conditions using nonparametric tests

Ameliorating ocean acidification: towards a model relating pCO2, irradiance and leaf area index of Zostera marina (eelgrass) in Padilla Bay, WA

In nearshore, soft-sediment habitats of the Salish Sea, eelgrass (Zostera marina L.) meadows have been identified as potential mitigators of ocean acidification (OA) because their photosynthetic activity can decrease pCO2, increase pH and provide refuge for organisms sensitive to OA. The diurnal light cycle controls photosynthetic production of eelgrass and therefore, along with tidal cycles, exerts strong controls on variations in pCO2 in nearshore environment. In this study, we investigate the carbon uptake rates for eelgrass under varying light, ambient pCO2 conditions and eelgrass densities (leaf area index). The magnitude of changes predicted based on experimentally derived photosynthetic rates, measured light and water depth in Padilla Bay, WA compare well with observed variability in the field. The ambient pCO2 conditions we tested, however, did not appear to be a major control in carbon uptake rates for eelgrass. Combining lab, model, and field results will strengthen our understanding of the variability of OA in the nearshore environment and help shellfish managers understand the drivers of that variability and inform further studies of its effects, such as potential OA refuge for shellfish and other sensitive organisms

Comparison of shallow-water seston among biogenic habitats on tidal flats

Aquatic structure-formers have the potential to establish mosaics of seston in shallow water if they modify the relative amounts of deposition (or filtration) and resuspension of particles. By sampling surface water adjacent to Lagrangian drifters traveling 0.1 to 2 m above the bottom, we tested the modification of seston in water masses flowing over two biogenic marine species (native eelgrass, Zostera marina; introduced oysters, Crassostrea gigas) in comparison to unstructured tidal flats. Water properties were examined at five intertidal sites in Washington State, USA, each with 27 drifts (three drifts at different stages of the tidal cycle in each of three patches of three habitat types; drift distance 116 m (109SD), duration 24 min (15SD)). At the initiation of each drift, habitat differences in water properties were already apparent: chlorophyll-a and total suspended solid (TSS) concentrations were greater in structured habitats than bare, and TSS was also inversely related to water depth. Water flowed more slowly across eelgrass than other habitat types. As water flowed across each habitat type, TSS generally increased, especially in shallow water, but without habitat differences; chlorophyll-a in these surface-water samples showed no consistent change during drifts. At higher TSS concentrations, quality in terms of organic content declined, and this relationship was not habitat-specific. However, quality in terms of chlorophyll-a concentration increased with TSS, as well as being greater in water over eelgrass than over other habitat types. These results support widespread mobilization of seston in shallow water ebbing or flooding across Washington State’s tidal flats, especially as water passes into patches of biogenic species

Mesopredator trait and eelgrass data from three sites in Washington State

This file includes raw count and length data for individual mesopredators collected in the field at three sites in Washington State, across three habitat types in three subsites per site. Except at the first subsite, ten individuals of each species were measured out of the total number counted. The file also includes functional traits assigned to each species observed in seine samples, using information freely available at FishBase and other sources detailed in the manuscript and supplementary material. Functional groups based on Gower distances were calculated using the FD package in R, and are also included here. Finally, we include data on eelgrass morphology including shoot density, biomass, leaf length, sheath length, and sheath width, as well as epiphyte load, measured in 10 0.25m^2 quadrats in eelgrass and edge habitat at each subsite