Advances in in situ imaging allow enumeration of abundant populations of large Rhizarians that compose a substantial proportion of total mesozooplankton biovolume. Using a quasi‐Lagrangian sampling scheme, we quantified the abundance, vertical distributions, and sinking‐related mortality of Aulosphaeridae, an abundant family of Phaeodaria in the California Current Ecosystem. Inter‐cruise variability was high, with average concentrations at the depth of maximum abundance ranging from < 10 to > 300 cells m−3, with seasonal and interannual variability associated with temperature‐preferences and regional shoaling of the 10°C isotherm. Vertical profiles showed that these organisms were consistently most abundant at 100–150 m depth. Average turnover times with respect to sinking were 4.7–10.9 d, equating to minimum in situ population growth rates of ~ 0.1–0.2 d−1. Using simultaneous measurements of sinking organic carbon, we find that these organisms could only meet their carbon demand if their carbon : volume ratio were ~ 1 μg C mm−3. This value is substantially lower than previously used in global estimates of rhizarian biomass, but is reasonable for organisms that use large siliceous tests to inflate their cross‐sectional area without a concomitant increase in biomass. We found that Aulosphaeridae alone can intercept > 20% of sinking particles produced in the euphotic zone before these particles reach a depth of 300 m. Our results suggest that the local (and likely global) carbon biomass of Aulosphaeridae, and probably the large Rhizaria overall, needs to be revised downwards, but that these organisms nevertheless play a major role in carbon flux attenuation in the twilight zone.
The summertime North Pacific subtropical gyre has widespread phytoplankton blooms between Hawaii and the subtropical front (∼30°N) that appear as chlorophyll (chl) increases in satellite ocean color data. Nitrogen-fixing diatom symbioses (diatom-diazotroph associations: DDAs) often increase 102–103 fold in these blooms and contribute to elevated export flux. In 2008 and 2009, two cruises targeted satellite chlorophyll blooms to examine DDA species abundance, chlorophyll concentration, biogenic silica concentration, and hydrography. Generalized observations that DDA blooms occur when the mixed layer depth is < 70 m are supported, but there is no consistent relationship between mixed layer depth, bloom intensity, or composition; regional blooms between 22–34°N occur within a broader temperature range (21–26°C) than previously reported. In both years, the Hemiaulus-Richelia and Rhizosolenia-Richelia DDAs increased 102–103 over background concentrations within satellite-defined bloom features. The two years share a common trend of Hemiaulus dominance of the DDAs and substantial increases in the >10 µm chl a fraction (∼40–90+% of total chl a). Integrated diatom abundance varied 10-fold over <10 km. Biogenic silica concentration tracked diatom abundance, was dominated by the >10 µm size fraction, and increased up to 5-fold in the blooms. The two years differed in the magnitude of the surface chl a increase (2009>2008), the abundance of pennate diatoms within the bloom (2009>2008), and the substantially greater mixed layer depth in 2009. Only the 2009 bloom had sufficient chl a in the >10 µm fraction to produce the observed ocean color chl increase. Blooms had high spatial variability; ocean color images likely average over numerous small events over time and space scales that exceed the individual event scale. Summertime DDA export flux noted at the Hawaii time-series Sta. ALOHA is probably a generalized feature of the eastern N. Pacific north to the subtropical front.
In the offshore waters of Southern California, submesoscale processes associated with fronts may stimulate phytoplankton blooms and lead to biomass shifts at multiple trophic levels. Here we report the results of a study on the cycling of biogenic silica (bSiO 2 ) with estimates of the contributions of diatoms to primary and new production in water masses adjacent to (i.e., coastal or oceanic) and within an offshore front in the Southern California Current Ecosystem (CCE). The coastal and oceanic water were sampled in cyclonic and anticyclonic eddies, respectively, with the frontal water being an interaction region between the eddy types. Concentrations of bSiO 2 varied by 25-fold across the front, with concentrations in frontal waters 20-25% of those in coastal waters. Rates of biogenic silica production spanned an equally large range, with rates within the frontal region that were half those in the coastal regions. Contributions of diatoms to primary and new production were disproportionately higher than their contribution to autotrophic biomass in all areas, ranging from 5-8%, 19-30%, and 32-43% for both processes in the oceanic, frontal and coastal waters, respectively. Across the frontal area, diatoms could account for <1.0%, 6-8%, and 44-72% of organic matter export in the oceanic, frontal and coastal waters, respectively. The results suggest that the regions of frontal interactions between eddies in the southern CCE can account for variability in diatom biomass, productivity and export over very short spatial scales that is comparable to the variability observed across the Pacific basin.
Nutrient dynamics, phytoplankton rate processes, and export were examined in a frontal region between an anticyclone and a pair of cyclones 120 km off the coast in the southern California Current System (sCCS). Low silicic acid: nitrate ratios (Si:N) and high nitrate to iron ratios (N:Fe) characteristic of Fe‐limiting conditions in the sCCS were associated with the northern cyclone and with the transition zone between the cyclones and the anticyclone. Phytoplankton growth in low‐Si:N, high‐N:Fe waters responded strongly to added Fe, confirming growth limitation by Fe of the diatom‐dominated phytoplankton community. Low Si:N waters had low biogenic silica content, intermediate productivity, but high export compared to intermediate Si:N waters indicating increased export efficiency under Fe stress. Biogenic silica and particulate organic carbon (POC) export were both high beneath low Si:N waters with biogenic silica export being especially enhanced. This suggests that relatively high POC export from low Si:N waters was supported by silica ballasting from Fe‐limited diatoms. Higher POC export efficiency in low Si:N waters may have been further enhanced by lower rates of organic carbon remineralization due to reduced grazing of more heavily armored diatoms growing under Fe stress. The results imply that Fe stress can enhance carbon export, despite lowering productivity, by driving higher export efficiency.
In marine ecosystems, many planktonic organisms precipitate biogenic silica (bSiO2) to build silicified skeletons. Among them, giant siliceous rhizarians (>500 μm), including Radiolaria and Phaeodaria, are important contributors to oceanic carbon pools but little is known about their contribution to the marine silica cycle. We report the first analyses of giant phaeodarians to bSiO2 export in the California Current Ecosystem. We measured the silica content of single rhizarian cells ranging in size from 470 to 3,920 μm and developed allometric equations to predict silica content (0.37–43.42 μg Si/cell) from morphometric measurements. Using sediment traps to measure phaeodarian fluxes from the euphotic zone on four cruises, we calculated bSiO2 export produced by two families, the Aulosphaeridae and Castanellidae. Biogenic silica export ranged from <0.01 to 0.63 mmol Si · m−2 · day−1. These two families alone contributed on average 10% (range 0–80%) of total bSiO2 export from the euphotic zone. Their proportional contributions increased substantially in more oligotrophic regions with lower bSiO2 fluxes. Using the in situ Underwater Vision Profiler 5, we characterized vertical distributions of the giant phaeodarian family Aulosphaeridae to a depth of 500 m and inferred their contribution to bSiO2 export in deeper waters. We found a significant increase of Aulosphaeridae export (<0.01 to 2.82 mmol Si · m−2 · day−1) when extended to mesopelagic depths. Using a global data set of in situ profiles, we estimated the significance of Aulosphaeridae to bSiO2 export and revealed that they can act as major exporters of bSiO2 to the mesopelagic zone in various regions.
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