An official website of the United States government
Abstract Giant kelp (Macrocystis pyrifera) forests are common along the California coast. Attached on the rocky bottom at depths of approximately 5–25 m, the kelp, when mature, spans the water column and develops dense, buoyant canopies that interact with waves and currents. We present two novel results based on observations of surface gravity waves in a kelp forest in Point Loma, California. First, we report short wave (1–3 s) attenuation in kelp, quantified by an exponential decay coefficient —comparable to the dampening effect of sea ice. Second, we identify seasonal and tidal changes in attenuation, peaking mid‐summer with maximum kelp cover, and during low tide when a greater proportion of the fronds are at the surface. Thus, the naturally occurring surface canopies of kelp forests can act as temporally varying, high‐frequency filters of wave energy.
more »
« less
Persistence of southern California giant kelp beds and alongshore variation in nutrient exposure driven by seasonal upwelling and internal waves
Kelp beds provide significant ecosystem services and socioeconomic benefits globally, and prominently in coastal zones of the California Current. Their distributions and abundance, however, vary greatly over space and time. Here, we describe long-term patterns of Giant Kelp (Macrocystis pyrifera) sea surface canopy area off the coast of San Diego County from 1983 through 2019 along with recent patterns of water column nitrate (NO3-) exposure inferred fromin situtemperature data in 2014 and 2015 at sites spanning 30 km of the coastline near San Diego California, USA. Site-specific patterns of kelp persistence and resilience were associated with ocean and climate dynamics, with total sea surface kelp canopy area varying approximately 33-fold over the almost 4 decades (min 0.34 km2in 1984; max 11.25 km2in 2008, median 4.79 km2). Site-normalized canopy areas showed that recent kelp persistence since 2014 was greater at Point Loma and La Jolla, the largest kelp beds off California, than at the much smaller kelp bed off Cardiff. NO3-exposure was estimated from an 11-month time series ofin situwater column temperature collected in 2014 and 2015 at 4 kelp beds, using a relationship between temperature and NO3-concentration previously established for the region. The vertical position of the 14.5°C isotherm, an indicator of the main thermocline and nutricline, varied across the entire water column at semidiurnal to seasonal frequencies. We use a novel means of quantifying estimated water column NO3-exposure integrated through time (mol-days m-2) adapted from degree days approaches commonly used to characterize thermal exposures. Water column integrated NO3-exposure binned by quarters of the time series showed strong seasonal differences with highest exposure in Mar - May 2015, lowest exposure in Sep - Dec 2014, with consistently highest exposure off Point Loma. The water column integrated NO3-signal was filtered to provide estimates of the contribution to total nitrate exposure from high frequency variability (ƒ >= 1 cycle 30 hr-1) associated predominantly with internal waves, and low frequency variability driven predominantly by seasonal upwelling. While seasonal upwelling accounted for > 90% of NO3-exposure across the full year, during warm periods when seasonal upwelling was reduced or absent and NO3-exposure was low overall, the proportion due to internal waves increased markedly to 84 to 100% of the site-specific total exposure. The high frequency variability associated with internal waves may supply critical nutrient availability during anomalously warm periods. Overall, these analyses support a hypothesis that differences in NO3-exposure among sites due to seasonal upwelling and higher frequency internal wave forcing contribute to spatial patterns in Giant Kelp persistence in southern California. The study period includes anomalously warm surface conditions and the marine heatwave associated with the “Pacific Warm Blob” superimposed on the seasonal thermal signal and corresponding to the onset of a multi-year decline in kelp canopy area and marked differences in kelp persistence among sites. Our analysis suggests that, particularly during periods of warm surface conditions, variation in NO3-exposure associated with processes occurring at higher frequencies, including internal waves can be a significant source of NO3-exposure to kelp beds in this region. The patterns described here also offer a view of the potential roles of seasonal and higher frequency nutrient dynamics for Giant Kelp persistence in southern California under continuing ocean surface warming and increasing frequency and intensity of marine heatwaves.
more »
« less
- PAR ID:
- 10480055
- Publisher / Repository:
- Frontiers Media S.A.
- Date Published:
- Journal Name:
- Frontiers in Marine Science
- Volume:
- 10
- ISSN:
- 2296-7745
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Two moorings equipped with autonomous air‐sea CO2instrumentation located in the Southern California Current System were used to examine the seasonal and interannual variability of the surface partial pressure of carbon dioxide in seawater (pCO2,sw) and the air‐sea CO2flux between 2008 and 2022. These two moorings are in two distinct oceanographic regimes: offshore, centered in the California Current (CCE1), and nearshore within the coastal upwelling regime (CCE2). The offshore seasonal cycles of the surfacepCO2,swand CO2flux are driven by sea surface temperature (SST) seasonality and at the nearshore site by dissolved inorganic carbon (DIC) concentration changes linked with seasonal upwelling. The resulting net annual CO2flux at CCE1 is −0.52 molC m−2 year−1(sink), while at CCE2, the best estimate for the long‐term CO2flux mean is 0.23 molC m−2 year−1(source). The interannual variability at the offshore site is mainly controlled by SST, where warm anomalies (El Niño and Marine Heatwaves) cause anomalous CO2outgassing, and cold anomalies (La Niña) increase CO2ingassing. Conversely, at the nearshore site, the strength (or absence of) upwelling of DIC‐rich water associated with cold (or warm anomalies) results in increased outgassing (or ingassing) of CO2. Long‐term trends inpCO2,swapproximately follow the atmospheric CO2increase. At the offshore site, the DIC trend is consistent with air‐sea fluxes, keeping the CO2equilibrium between air and water. At the nearshore site, the DIC trend has a similar magnitude but could also result from changing water‐mass composition or concentration due to freshwater loss.more » « less
-
Abstract Increased ocean temperatures have led to large‐scale declines in many ecologically important species, including kelp forests. Spatial heterogeneity across seascapes could protect kelp individuals and small populations from thermal stress and nutrient limitation. Habitat features within upwelling regions may facilitate the transport of deep, cold water into shallow systems, but little is known about the spatiotemporal occurrence or stability of these climate refugia. Kelp in climate refugia may, however, also experience other stressors, such as overgrazing by kelp herbivores, reducing their effectiveness.Here, we use high‐resolution kelp canopy maps generated from CubeSat constellation data to characterize kelp persistence in northern California following a dramatic decline in kelp abundance due to increased temperature and nutrient limitation during a severe marine heatwave and continued intense grazing pressure by purple sea urchins.Kelp persistence was associated with local areas of relatively cool water temperature and seascape features such as shallow depths and low‐complexity bathymetry, which may have provided refuge from overgrazing. However, a very small percentage of kelp forests in the region exhibited high persistence, with many forests present in only one or two of the 9 years studied. Most kelp patches were not spatially stable over time. Initially, kelp presence aligned with climate refugia, but as overgrazing emerged as the dominant driver of kelp distributions post‐2019, kelp shifted to areas that offered protection from grazing pressure.Synthesis. Cooler areas with localized upwelling acted as climate refugia during the increased ocean temperatures from the 2014–2016 marine heatwave, supporting nutrient‐rich environments and mitigating heat stress for kelp forests. However, these temperature refugia often did not spatially overlap with areas providing protection from grazing pressure, leaving kelp forests vulnerable to future warming even within temperature refugia if grazing pressure remains high.more » « less
-
Vital rates, including growth responses to environmental variability, are poorly characterized for the diverse taxa of heterotrophic bacteria (HBact) in marine ecosystems. Here, we evaluated the potential for combining molecular analyses with dilution experiments to assess taxon-specific growth (cell division) and net growth rates of HBact in natural waters. Two-treatment dilution experiments were conducted within situincubations under 3 environmental conditions in the California Current Ecosystem, at offshore and inshore sites during a warm upwelling-suppressed year (2014) and for normal inshore upwelling, representing a 33-fold primary production range. Relative sequence reads from 16S rRNA metabarcoding were normalized to total HBact counts from flow cytometry for community abundance and rate calculations. Composition varied from dominance of Alphaproteobacteria (56%) in oligotrophic offshore (SAR11) and mesotrophic inshore waters (SAR11 and Rhodobacteria) to Bacteriodes/Flavobacteria dominance (64%) and mixed sub-taxon importance (Polaribacter, Rhodobacteria,Formosa) during upwelling. Net growth rates in bottles, validated by comparison to ambient community net growth following a satellite-tracked drifter, varied from near steady state for offshore and inshore conditions to dynamic community changes during upwelling. Mean growth rates doubled (0.33 to 0.62 d-1) over the productivity range, and taxon estimates varied from -0.17 d-1(Formosa, offshore) to 1.53 d-1(SAR11, upwelling). Increasing growth of Flavobacteria and Rhodobacteria paralleled their abundance and dominance increases with productivity. SAR11 growth remained higher than average with increasing production, despite declining abundances. We highlight possible PCR or 16S rRNA gene copy biases of growth rate estimates as research needs for further applications of this approach.more » « less
-
Abstract The distributions of iodate and iodide were measured along the GEOTRACES GP15 meridional transect at 152°W from the shelf of Alaska to Papeete, Tahiti. The transect included oxygenated waters near the shelf of Alaska, the full water column in the central basin in the North Pacific Basin, the upper water column spanning across seasonally mixed regimes in the north, oligotrophic regimes in the central gyre, and the equatorial upwelling. Iodide concentrations are highest in the permanently stratified tropical mixed layers, which reflect accumulation due to light‐dependent biological processes, and decline rapidly below the euphotic zone. Vertical mixing coefficients (Kz), derived from complementary7Be data, enabled iodide oxidation rates to be estimated at two stations. Iodide half‐lives of 3–4 years show the importance of seasonal mixing processes in explaining north‐south differences in the transect, and also contribute to the decrease in iodide concentrations with depth below the mixed layer. These estimated half‐lives are consistent with a recent global iodine model. No evidence was found for significant inputs of iodine from the Alaskan continental margin, but there is a significant enrichment of iodide in bottom waters overlying deep sea sediments from the interior of the basin.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3389/fmars.2023.1007789
