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

Abstract

Abstract. Predicting consequences of ocean deoxygenation and ocean acidification for nearshore marine ecosystems requires baseline dissolved oxygen (DO) and carbonate chemistry data that are both high-frequency and high-quality. Such data allow accurate assessment of environmental variability and present-day organism exposure regimes. In this study, scales of DO and pH variability were characterized over one year in a nearshore kelp forest ecosystem in the Southern California Bight. DO and pH were strongly, positively correlated, revealing that organisms on this upwelling shelf are not only exposed to low pH but also to low DO. The dominant scale of temporal DO and pH variability occurred 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 as 220 µmol kg −1 and 0.36 units, respectively. Sources of pH and DO variation include photosynthesis within the kelp forest ecosystem, which can elevate DO and pH by up to 60 µmol kg −1 and 0.1 units over one week following the intrusion of high-density, nutrient-rich water. Accordingly, highly productive macrophyte-based ecosystems could serve as deoxygenation and acidification refugia by acting to elevate DO and pH relative to surrounding waters. DO and pH exhibited greater spatial variation over a 10 m increase in water depth (from 7 to 17 m) than along a 5 km stretch of shelf in a cross-shore or alongshore direction. Over a threemonth time period, mean DO and pH at 17 m water depth were 168 µmol kg −1 and 7.87, respectively. These values represent a 35 % decrease in mean DO and 37 % increase in [H + ] relative to near-surface waters. High-frequency variation was also reduced at depth. The mean 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 history of an organism is largely a function of its depth of occurrence within the kelp forest. With knowledge of local alkalinity conditions and high-frequency temperature, salinity, and pH data, we estimated pCO 2 and calcium carbonate saturation states with respect to calcite and aragonite ( calc and arag ) for the La Jolla kelp forest at 7 m and 17 m water depth. pCO 2 ranged from 246 to 1016 µatm, calc was always supersaturated, and arag was undersaturated at the beginning of March for five days when pH was less than 7.75 and DO was less than 115 µmol kg −1 . These findings raise the possibility that the benthic communities along eastern boundary current systems are currently acclimatized and adapted to natural, variable, and low DO and pH. Still, future exposure of coastal California populations to even lower DO and pH may increase as upwelling intensifies and hypoxic boundaries shoal, compressing habitats and challenging the physiological capacity of intolerant species.

Historical baselines and the future of shell calcification for a foundation species in a changing ocean

“…The decreases in pH documented at Tatoosh Island since 2000 [25,67] and the low pH and aragonite saturation states seen in California [68,69] suggest that carbon cycle changes in the ocean are producing an environment where aragonite and calcite are more difficult for organisms to accrete.…”

CO2-induced ocean acidification increases anxiety in Rockfish via alteration of GABAAreceptor functioning

“…For example, while the pH of seawater is normally approximately 8.05 units and pCO 2 is approximately 400 matm, they can reach approximately 7.70 pH units and 1016 matm for several days [22] (approx. 5 days in that particular study, but as long as 11 days in other surveys that estimated upwelling from dissolved O 2 [47]).…”

“…Shortterm fluctuations in ocean dynamics can create localized areas of low pH (approx. 7.8) and higher than normal pCO 2 that can reach as high as 820 matm at shallow depths (7 m) and as high as 1016 matm at greater depths (17 m) for at least 5-7 days [22]. Therefore, the California coast already regularly experiences CO 2 -acidified water and will be exposed to more extreme OA conditions earlier than in other locations [23,24].…”

“…Non-reef fish that live near kelp forest ecosystems are candidates to be especially vulnerable because it has recently been demonstrated that upwelling events can significantly affect seawater chemistry [22]. Shortterm fluctuations in ocean dynamics can create localized areas of low pH (approx.…”

Sea HareAplysia punctata(Mollusca: Gastropoda) Can Maintain Shell Calcification under Extreme Ocean Acidification

“…In tidepools, short-term variation in pH can be considerable (up to 1.78 units) due to diurnal variations in calcification, net primary production, and temperature (Kwiatkowski et al, 2016). In upwelling areas, pH can vary diurnally by up to 0.36 (Frieder et al, 2012). In a Baltic fjord close to where the present study was conducted, the seasonal variability was ϳ0.6, with considerable changes of 0.3-0.4 occurring over timescales of days (Thomsen et al, 2010;Melzner et al, 2013).…”