Abstract The California Current System displays a strong seasonal cycle in water properties, circulation, and biological production. Interactions of the alongshore current with coastal and topographic features lead to high spatial variability forced by seasonal winds that displace surface coastal water offshore. This process also supplies nutrients to the euphotic zone by Ekman transport and eventually supports phytoplankton blooms typically dominated by diatoms. Here, we investigate the relationship between biogenic silica production and mesoscale upwelling dynamics along the central region of the California Current System between 2013 and 2015, a period affected by a warm anomaly known as “the Blob.” Changes in the upwelling phenology along California caused by this marine heatwave are investigated using an innovative index and related to patterns of diatom production during upwelling events to evaluate diatom resilience. Based on this new index, we estimated that the nutrient supply to the euphotic zone declined by 50% during the Blob, but the Blob had little impact on local production during individual upwelling events. A statistical analysis evaluating the relationship between production and environmental conditions reveals persistent biological hotspots characterized by high biomass, depleted nutrients, and high specific production rates (up to 0.7 d −1 ) throughout the study period. Lower observed biogenic silica to Chlorophyll a ratios during the Blob suggested a taxonomic shift from siliceous to nonsiliceous phytoplankton and/or lightly silicified diatoms signaling a change at the base of the food chain that could have ramifications for productivity in this eastern boundary coastal upwelling system.
The effects of added Si and Fe on the rate of silicic acid uptake were examined during two cruises to the equatorial Pacific upwelling zone between 110°W and 140°;W. Maximum uptake rates of Si ( V max ) were highly consistent with a mean of 0.026 ± 0.007 h −1 (n = 29), implying maximum diatom growth rates of ~0.6 d −1 . Half‐saturation constants for Si uptake ( K S ) also showed little variance, averaging 1.7 ± 0.7 mmol L −1 Si(OH) 4 . No ecologically significant spatial or temporal patterns for either V max or K S were observed. Comparison of Si uptake rates at the ambient silicic acid concentration ( V amb ) with V max indicated that the ambient [Si(OH) 4 ] restricted V amb to an average of 63% ± 13% of V max . Fe additions also caused significant increases in both V max and V amb , indicating that the rate of Si uptake was also regulated by the ambient [Fe]. Fe additions had a variable effect on K S , but they consistently increased both V max and the initial slope of the kinetic curve ( V max : K S ), and thus the diatom assemblages’ ability to take up Si(OH) 4 at low concentrations. Added Fe or Si increased Si uptake rates by 87% ± 59% and 69% ± 31%, respectively, indicating nearly equal roles for the two elements in limiting rates of Si uptake in situ. The largest average increase in Si uptake rates, 172% ± 43%, was observed when both Si and Fe were added, implying that together Si and Fe restricted Si uptake rates by almost a factor of three.
It is increasingly apparent that adequately mitigating anthropogenic climate interference will require ocean carbon dioxide removal (CDR) strategies. Ocean alkalinity enhancement (OAE) is an abiotic ocean CDR approach that aims to increase the ocean’s CO 2 uptake capacity through the dispersal of pulverized mineral or dissolved alkali into the surface ocean. However, OAE’s effect on marine biota is largely unexplored. Here, we investigate the impacts of moderate (~700 μmol kg −1 ) and high (~2700 μmol kg −1 ) limestone-inspired alkalinity additions on two biogeochemically and ecologically important phytoplankton functional group representatives: Emiliania huxleyi (calcium carbonate producer) and Chaetoceros sp. (silica producer). The growth rate and elemental ratios of both taxa showed a neutral response to limestone-inspired alkalinization. While our results are encouraging, we also observed abiotic mineral precipitation, which removed nutrients and alkalinity from solution. Our findings offer an evaluation of biogeochemical and physiological responses to OAE and provide evidence supporting the need for continued research into how OAE strategies affect marine ecosystems.
Coccolithophores are an important group of calcifying marine phytoplankton. Although coccolithophores are not silicified, some species exhibit a requirement for Si in the calcification process. These species also possess a novel protein (SITL) that resembles the SIT family of Si transporters found in diatoms. However, the nature of Si transport in coccolithophores is not yet known, making it difficult to determine the wider role of Si in coccolithophore biology. Here, we show that coccolithophore SITLs act as Na+ -coupled Si transporters when expressed in heterologous systems and exhibit similar characteristics to diatom SITs. We find that CbSITL from Coccolithus braarudii is transcriptionally regulated by Si availability and is expressed in environmental coccolithophore populations. However, the Si requirement of C. braarudii and other coccolithophores is very low, with transport rates of exogenous Si below the level of detection in sensitive assays of Si transport. As coccoliths contain only low levels of Si, we propose that Si acts to support the calcification process, rather than forming a structural component of the coccolith itself. Si is therefore acting as a micronutrient in coccolithophores and natural populations are only likely to experience Si limitation in circumstances where dissolved silicon (DSi) is depleted to extreme levels.
Silicon isotope tracers were used to examine the relative magnitude of silica dissolution and silica production in the Monterey Bay, California, upwelling system. A diatom bloom dominated by Skeletonema costatum and Chaetoceros spp. was encountered under conditions of moderate upwelling. Profiles of silica production and dissolution rates were obtained at seven stations that sampled both inside and outside the bloom. Integrated silica production rates ranged from 5.4 to 108 mmol Si m −2 d −1 , averaging 42.8 mmol Si m −2 d −1 . Integrated silica dissolution rates were considerably lower than production rates with values between 0.63 and 6.5 mmol Si m −2 d −1 (mean = 2.90 mmol Si m −2 d −1 ). The mean ratio of integrated silica dissolution to integrated silica production (∫ D:∫ P) between the surface and the 0.1% light depth was 0.075, omitting one station with an unusually high ∫ D:∫ P of 0.61, indicating that, on average, 93% of silica production was supported by new silicic acid. The f‐ratio for diatom nitrogen use estimated from silicic acid and nitrate depletion curves and the mean ∫ D:∫ P ratio was found to be 0.83, indicating that silica was being regenerated at a rate that was only slightly slower than that for particulate organic nitrogen. These data provide direct evidence confirming earlier hypotheses that the silica pump is weak in Monterey Bay. Analysis of the global data set on ∫ D:∫ P in the surface ocean leads to the hypothesis that low ∫ D: ∫ P (~0.10 or less) are typical of diatom bloom events, with ∫ D:∫ P rising to values in excess of 0.50 during nonbloom periods. This pattern is shown to be consistent with previous estimates that the annual mean ∫ D:∫ P ratio in the upper 200 m of the global ocean is 0.5–0.6. A regional analysis reveals that the fraction of silica production supported by new silicic acid varies as a hyperbolic function of the level of gross silica production similar to the variation in the f‐ratio for N use with primary productivity. These trends suggest that diatom blooms, especially those occurring in more productive waters, are the main vectors of silica export in the sea, with the majority of the silica produced during nonbloom periods being recycled in the euphotic zone.
