High temporal and spatial variability of dissolved oxygen and pH in a nearshore California kelp forest

摘要

Predicting consequences of ocean deoxygenation and ocean acidification fornearshore marine ecosystems requires baseline dissolved oxygen (DO) andcarbonate chemistry data that are both high-frequency and high-quality. Suchdata allow accurate assessment of environmental variability and present-dayorganism exposure regimes. In this study, scales of DO and pH variabilitywere characterized over one year in a nearshore kelp forest ecosystem in theSouthern California Bight. DO and pH were strongly, positively correlated,revealing that organisms on this upwelling shelf are not only exposed to lowpH but also to low DO. The dominant scale of temporal DO and pH variabilityoccurred on semidiurnal, diurnal and event (days–weeks) time scales.Daily ranges in DO and pH at 7 m water depth (13 mab) could be as large as220 μmol kg?1 and 0.36 units, respectively. Sources of pH andDO variation include photosynthesis within the kelp forest ecosystem, whichcan elevate DO and pH by up to 60 μmol kg?1 and 0.1 unitsover one week following the intrusion of high-density, nutrient-rich water.Accordingly, highly productive macrophyte-based ecosystems could serve asdeoxygenation and acidification refugia by acting to elevate DO and pHrelative to surrounding waters. DO and pH exhibited greater spatial variationover a 10 m increase in water depth (from 7 to 17 m) than along a 5 kmstretch of shelf in a cross-shore or alongshore direction. Over a three-monthtime period, mean DO and pH at 17 m water depth were168 μmol kg?1 and 7.87, respectively. These values representa 35% decrease in mean DO and 37% increase in [H+] relative tonear-surface waters. High-frequency variation was also reduced at depth. Themean daily range in DO and pH was 39% and 37% less, respectively,at 17 m water depth relative to 7 m. As a consequence, the exposure historyof an organism is largely a function of its depth of occurrence within thekelp forest. With knowledge of local alkalinity conditions and high-frequencytemperature, salinity, and pH data, we estimated pCO₂ and calciumcarbonate saturation states with respect to calcite and aragonite(Ω_calc and Ω_arag) for the La Jolla kelp forestat 7 m and 17 m water depth. pCO₂ ranged from 246 to1016 μatm, Ω_calc was always supersaturated, andΩ_arag was undersaturated at the beginning of March for fivedays when pH was less than 7.75 and DO was less than115 μmol kg?1. These findings raise the possibility that thebenthic communities along eastern boundary current systems are currentlyacclimatized and adapted to natural, variable, and low DO and pH. Still,future exposure of coastal California populations to even lower DO and pH mayincrease as upwelling intensifies and hypoxic boundaries shoal, compressinghabitats and challenging the physiological capacity of intolerant species.