Along upwelling margins, pH and oxygen vary on multiple temporal and spatial scales, and in many places levels are decreasing with climate change. Continuous monitoring in nearshore settings along an upwelling-influenced margin revealed strong, semidiurnal fluctuations, week-long reduction events, and a tight positive correlation between oxygen and pH. Laboratory experiments were conducted to assess implications of pH and oxygen changes for invertebrate gamete and larval performance. At levels reflecting nearshore conditions, there were effects of low pH on fertilization success in echinoids and larval development and size of two Mytilus mussel species, but there was no apparent effect of low oxygen alone or in combination with pH. Fertilization experiments indicated that pH variability present within the habitat of Strongylocentrotus franciscanus could hinder fertilization success when timing of spawning coincides with low pH conditions. The incorporation of semidiurnal pH fluctuations, the dominant scale of observed temporal variability, into laboratory experiments alleviated negative effects of reduced pH in both Mytilus species studied. Furthermore, at lower pH, high variance in echinoid sperm performance and in larval size of Mytilus spp. suggests the raw material exists for evolutionary adaptation to reduced pH. Population variance in combination with temporal and spatial variation in pH may be increasingly important in future, low-pH oceans. Additionally, the observation of species-specific responses to pH among congeneric echinoids and mytilid mussels implies that we cannot assume similar sensitivity to reduced pH based on taxonomic relatedness. Further understanding of responses to ocean acidification may be aided by knowledge of larval pH-exposure history. The development of a larval-based geochemical proxy revealed that U/Ca in larval shells reflected differing pH exposures of mussel larvae. Application to outplanted larvae developing along the San Diego coastline demonstrated that higher U/Ca in larval shells can reflect upwelling and exposure to low pH. Notably, present-day pH conditions are at times low enough to elicit significant effects on fertilization in S. franciscanus, on larval development of Mytilus spp., and on the geochemical composition of larval shells. These effects could influence the sustainability and persistence of these commercially harvested species as ocean acidification intensifies along upwelling margins
Abstract A key control on the magnitude of coastal eutrophication is the degree to which currents quickly transport nitrogen away from the coast to the open ocean before eutrophication symptoms develop. In the Southern California Bight (SCB), an upwelling-dominated eastern boundary current ecosystem, anthropogenic nitrogen inputs increase algal productivity and cause subsurface acidification and deoxygenation along the coast. However, the extent of anthropogenic influence beyond the coastal band, and the physical transport mechanisms responsible these effects, were not previously documented. Here, we use a submesoscale-resolving numerical model to investigate the transport of anthropogenic nitrogen and its effects on SCB offshore habitats. Anthropogenic nutrient inputs promoted an increase in productivity and respiration offshore, with recurrent oxygen loss and pH decline in a region located 30 – 90 km from the mainland. Over 2013 to 2017, peak losses up to 14.2 mmol m-3 O2 persisted 4 to 6 months of the year over an area of 278,400 km2 (~30\% of SCB area). These recurrent features are associated with eddy cross-shore transport of nutrients and plankton biomass, and their accumulation and retention within persistent eddies offshore counteract the dilution and dispersion by mean currents that transport nitrogen and organic matter out of the SCB.
Abstract Physiological increases in energy expenditure frequently occur in response to environmental stress. Although energy limitation is often invoked as a basis for decreased calcification under ocean acidification, energy-relevant measurements related to this process are scant. In this study we focus on first-shell (prodissoconch I) formation in larvae of the Pacific oyster, Crassostrea gigas. The energy cost of calcification was empirically derived to be ≤ 1.1 µJ (ng CaCO3)−1. Regardless of the saturation state of aragonite (2.77 vs. 0.77), larvae utilize the same amount of total energy to complete first-shell formation. Even though there was a 56% reduction of shell mass and an increase in dissolution at aragonite undersaturation, first-shell formation is not energy limited because sufficient endogenous reserves are available to meet metabolic demand. Further studies were undertaken on larvae from genetic crosses of pedigreed lines to test for variance in response to aragonite undersaturation. Larval families show variation in response to ocean acidification, with loss of shell size ranging from no effect to 28%. These differences show that resilience to ocean acidification may exist among genotypes. Combined studies of bioenergetics and genetics are promising approaches for understanding climate change impacts on marine organisms that undergo calcification.
Exogenous environmental factors alter growth rates, yet information remains scant on the biochemical mechanisms and energy trade-offs that underlie variability in the growth of marine invertebrates. Here we study the biochemical bases for differential growth and energy utilization (as adenosine triphosphate [ATP] equivalents) during larval growth of the bivalve Crassostrea gigas exposed to increasing levels of experimental ocean acidification (control, middle, and high pCO2, corresponding to ∼400, ∼800, and ∼1100 µatm, respectively). Elevated pCO2 hindered larval ability to accrete both shell and whole-body protein content. This negative impact was not due to an inability to synthesize protein per se, because size-specific rates of protein synthesis were upregulated at both middle and high pCO2 treatments by as much as 45% relative to control pCO2. Rather, protein degradation rates increased with increasing pCO2. At control pCO2, 89% of cellular energy (ATP equivalents) utilization was accounted for by just 2 processes in larvae, with protein synthesis accounting for 66% and sodium-potassium transport accounting for 23%. The energetic demand necessitated by elevated protein synthesis rates could be accommodated either by reallocating available energy from within the existing ATP pool or by increasing the production of total ATP. The former strategy was observed at middle pCO2, while the latter strategy was observed at high pCO2. Increased pCO2 also altered sodium-potassium transport, but with minimal impact on rates of ATP utilization relative to the impact observed for protein synthesis. Quantifying the actual energy costs and trade-offs for maintaining physiological homeostasis in response to stress will help to reveal the mechanisms of resilience thresholds to environmental change.
Abstract Chemical properties of the California Undercurrent (CU) have been changing over the past several decades, yet the mechanisms responsible for the trend are still not fully understood. We present a survey of temperature, salinity, O 2 , pH, and currents at intermediate depths (defined here as 50–500 m) in the summer (30 June to 10 July) and winter (8–15 December) of 2012 in the southern region of the Southern California Bight. Observations of temperature, salinity, and currents reveal that local bathymetry and small gyres play an important role in the flow path of the California Undercurrent (CU). Using spiciness (π) as a tracer, we observe a 10% increase of Pacific Equatorial Water (PEW) in the core of the CU during the summer versus the winter. This is associated with an increase in π of 0.2, and a decrease in O 2 and pH of 30 μmol kg −1 and 0.022, respectively; the change in pH is driven by increased CO 2 , while total alkalinity remains unchanged. The high‐π, low‐O 2 , and low‐pH waters during the summer are not distributed uniformly in the study region. Moreover, mooring observations at the edge of the continental shelf reveal intermittent intrusions of PEW onto the shelf with concomitant decreases in O 2 and pH. We estimate that increased advection of PEW in the CU could account for approximately 50% of the observed decrease in O 2 , and between 49 and 73% of the decrease in pH, over the past three decades.
A key control on the magnitude of coastal eutrophication is the degree to which currents quickly transport nitrogen derived from human sources away from the coast to the open ocean before eutrophication develops. In the Southern California Bight (SCB), an upwelling-dominated eastern boundary current ecosystem, anthropogenic nitrogen inputs increase algal productivity and cause subsurface acidification and oxygen (O
International climate goals require over 5 gigatons/year (Gt/year) of CO2 to be removed from the atmosphere by midcentury. Macroalgae mariculture has been proposed as a strategy for such carbon dioxide removal (CDR). However, the global potential for seaweed cultivation has not been assessed in detail. Here, we develop and use a dynamic seaweed growth model, the Global MacroAlgae Cultivation MODeling System (G-MACMODS), to estimate potential yields of four different types of seaweed worldwide, and test the sensitivity of these estimates to uncertain biophysical parameters under two nutrient scenarios (one in which the surface ocean nutrient budget is unaltered by the presence of seaweed farms, and another in which seaweed harvest is limited by nutrients that are resupplied by vertical transport). We find that 1 Gt of seaweed carbon could be harvested in 0.8% of global exclusive economic zones (EEZs; equivalent to ~1 million km2) if farms were located in the most productive areas, but potential harvest estimates are highly uncertain due to ill-constrained seaweed mortality and nitrogen exudation rates. Our results suggest that seaweed farming could produce climate-relevant quantities of biomass carbon and highlight key uncertainties to be resolved by future research.
