skip to main content

This content will become publicly available on December 21, 2022

Title: Limited biogeochemical modification of surface waters by kelp forest canopies: Influence of kelp metabolism and site‐specific hydrodynamics
Climate change is causing decreases in pH and dissolved oxygen (DO) in coastal ecosystems. Canopy-forming giant kelp can locally increase DO and pH through photosynthesis, with the most pronounced effect expected in surface waters where the bulk of kelp biomass resides. However, limited observations are available from waters in canopies and measurements at depth show limited potential of giant kelp to ameliorate chemical conditions. We quantified spatiotemporal variability of surface biogeochemistry and assessed the role of biological and physical drivers in pH and DO modification at two locations differing in hydrodynamics inside and outside of two kelp forests in Monterey Bay, California in summer 2019. pH, DO, dissolved inorganic carbon (DIC), and temperature were measured at and near the surface, in conjunction with physical parameters (currents and pressure), nutrients, and metrics of phytoplankton and kelp biological processes. DO and pH were highest, with lower DIC, at the surface inside kelp forests. However, differences inside vs. outside of kelp forests were small (DO 6–8%, pH 0.05 higher in kelp). The kelp forest with lower significant wave height and slower currents had greater modification of surface biogeochemistry as indicated by larger diel variation and slightly higher mean DO and pH, despite lower more » kelp growth rates. Differences between kelp forests and offshore areas were not driven by nutrients or phytoplankton. Although kelp had clear effects on biogeochemistry, which were modulated by hydrodynamics, the small magnitude and spatial extent of the effect limits the potential of kelp forests to mitigate acidification and hypoxia. « less
; ; ; ; ; ; ; ; ; ; ;
Award ID(s):
1737176 1737096
Publication Date:
Journal Name:
Limnology and Oceanography
Sponsoring Org:
National Science Foundation
More Like this
  1. The United States Department of Energy (DOE)’s Ocean Margins Program (OMP) cruise EN279 in March 1996 provides an important baseline for assessing long-term changes in the carbon cycle and biogeochemistry in the Mid-Atlantic Bight (MAB) as climate and anthropogenic changes have been substantial in this region over the past two decades. The distributions of O 2 , nutrients, and marine inorganic carbon system parameters are influenced by coastal currents, temperature gradients, and biological production and respiration. On the cross-shelf direction, pH decreases seaward, but carbonate saturation state (Ω Arag ) does not exhibit a clear trend. In contrast, Ω Arag increases from north to south, while pH has no clear spatial patterns in the along-shelf direction. In order to distinguish between the effects of physical mixing of various water masses and those of biological activities on the marine inorganic carbon system, we use the potential temperature-salinity diagram to identify water masses, and differences between observations and theoretical mixing concentrations to measure the non-conservative (primarily biological) effects. Our analysis clearly shows the degree to which ocean margin pH and Ω Arag are regulated by biological activities in addition to water mass mixing, gas exchange, and temperature. The correlations among anomalies inmore »dissolved inorganic carbon, phosphate, nitrate, and apparent oxygen utilization agree with known biological stoichiometry. Biological uptake is substantial in nearshore waters and in shelf-slope mixing areas. This work provides valuable baseline information to assess the more recent changes in the marine inorganic carbon system and the status of coastal ocean acidification.« less
  2. Abstract

    Kelp forests support some of the most productive and diverse ecosystems on Earth, and their ability to uptake dissolved inorganic carbon (DIC) allows them to modify local seawater chemistry, creating gradients in carbon, pH, and oxygen in their vicinity. By taking up both bicarbonate and CO2as a carbon source for photosynthesis, kelp forests can act as carbon sinks, reducing nearby acidity and increasing dissolved oxygen; creating conditions conducive to calcification. Recent stressors, however, have reduced kelp forest canopies globally; converting once large and persistent forests to fragmented landscapes of small kelp patches. In a two-year study, we determined whether fragmented kelp patches retained the ability to alter local seawater chemistry. We found that diel fluctuations of multiple parameters of carbonate chemistry were greater in the kelp canopy than in the kelp benthos and in adjacent urchin barrens, consistent with metabolic activity by the kelp. Further, kelp fragments increased pH and aragonite saturation and decreasedpCO2during the day to a similar degree as large, intact kelp forests. We conclude that small kelp patches could mitigate OA stress and serve as spatial and temporal refugia for canopy-dwelling organisms, though this effect is temporary and confined to daylight hours during the growing season.

  3. Abstract. Global projections for ocean conditions in 2100 predict that the North Pacific will experience some of the largest changes. Coastal processes that drive variability in the region can alter these projected changes but are poorly resolved by global coarse-resolution models. We quantify the degree to which local processes modify biogeochemical changes in the eastern boundary California Current System (CCS) using multi-model regionally downscaled climate projections of multiple climate-associated stressors (temperature, O2, pH, saturation state (Ω), and CO2). The downscaled projections predict changes consistent with the directional change from the global projections for the same emissions scenario. However, the magnitude and spatial variability of projected changes are modified in the downscaled projections for carbon variables. Future changes in pCO2 and surface Ω are amplified, while changes in pH and upper 200 m Ω are dampened relative to the projected change in global models. Surface carbon variable changes are highly correlated to changes in dissolved inorganic carbon (DIC), pCO2 changes over the upper 200 m are correlated to total alkalinity (TA), and changes at the bottom are correlated to DIC and nutrient changes. The correlations in these latter two regions suggest that future changes in carbon variables are influenced by nutrient cycling, changes inmore »benthic–pelagic coupling, and TA resolved by the downscaled projections. Within the CCS, differences in global and downscaled climate stressors are spatially variable, and the northern CCS experiences the most intense modification. These projected changes are consistent with the continued reduction in source water oxygen; increase in source water nutrients; and, combined with solubility-driven changes, altered future upwelled source waters in the CCS. The results presented here suggest that projections that resolve coastal processes are necessary for adequate representation of the magnitude of projected change in carbon stressors in the CCS.« less
  4. Satellite and aerial imagery have been used extensively for mapping the abundance and distribution of giant kelp ( Macrocystis pyrifera ) in southern California. There is now great potential for using unoccupied aerial vehicles (UAVs) to map kelp canopy at very high resolutions. However, tides and currents have been shown to affect the amount of floating kelp canopy on the water surface, and the impacts of these processes on remotely sensed kelp estimates in this region have not been fully quantified. UAVs were used to map fine-scale changes in canopy area due to tidal height and current speed at kelp forests off the coast of Palos Verdes, CA and Santa Barbara, CA. An automated method for detecting kelp canopy was developed that was 67% accurate using red-green-blue (RGB) UAV imagery and 93% accurate using multispectral UAV imagery across a range of weather, ocean, and illumination conditions. Increases in tidal height of 1 m reduced the amount of floating kelp canopy by 15% in Santa Barbara and by over 30% in Palos Verdes. The effect of current speed on visible kelp canopy was inconclusive, but there was a trend towards lower canopy area with increased current speed. Therefore, while tidal height andmore »current speed can introduce significant variability to estimates of kelp abundance, the magnitude of this variability is site specific. Still, UAVs are a valuable tool for mapping of kelp canopy and can provide greater spatial resolution and temporal coverage than is possible from many satellite sensors. This data can provide insight into the patterns and drivers of high frequency fluctuations in kelp abundance.« less
  5. Geological sources of methane (CH4), such as hydrocarbon seeps, are significant yet poorly constrained sources of CH4 to seawater and the overlying atmosphere. We investigate the radiocarbon content (14C) and concentrations of dissolved CH4 in surface waters from the Coal Oil Point seep field to test the hypothesis that geological sources can dominate the regional background signal of CH4. We find that surface waters with elevated CH4 concentration were populated with seep-CH4 and that lower concentrations of CH4 were well explained by mixing with the regional background of non-geological CH4. Substantial differences in concentration and 14C-CH4 were observed over distances <5 km, demonstrating that surface currents mix background-CH4 into the seep field. These results indicate that even a prolific seep region like the Santa Barbara Basin exerts limited influence on the regional background of CH4 in the surface layer but is a significant driver of patchiness in oceanic CH4 biogeochemistry.