Plankton dynamics and biogeochemical fluxes in the Costa Rica Dome: introduction to the CRD Flux and Zinc Experiments

Differential Distribution and Determinants of Ammonia Oxidizing Archaea Sublineages in the Oxygen Minimum Zone off Costa Rica

Ammonia oxidizing archaea (AOA) are microbes that are widely distributed in the ocean that convert ammonia to nitrite for energy acquisition in the presence of oxygen. Recent study has unraveled highly diverse sublineages within the previously defined AOA ecotypes (i.e., water column A (WCA) and water column B (WCB)), although the eco-physiology and environmental determinants of WCB subclades remain largely unclear. In this study, we examined the AOA communities along the water columns (40-3000 m depth) in the Costa Rica Dome (CRD) upwelling region in the eastern tropical North Pacific Ocean. Highly diverse AOA communities that were significantly different from those in oxygenated water layers were observed in the core layer of the oxygen minimum zone (OMZ), where the dissolved oxygen (DO) concentration was < 2μM. Moreover, a number of AOA phylotypes were found to be enriched in the OMZ core. Most of them were negatively correlated with DO and were also detected in other OMZs in the Arabian Sea and Gulf of California, which suggests low oxygen adaptation. This study provided the first insight into the differential niche partitioning and environmental determinants of various subclades within the ecotype WCB. Our results indicated that the ecotype WCB did indeed consist of various sublineages with different eco-physiologies, which should be further explored.

Microplankton trace element contents: implications for mineral limitation of mesozooplankton in an HNLC area

Mesozooplankton production in high-nutrient low-chlorophyll regions of the ocean may be reduced if the trace element concentrations in their food are insufficient to meet growth and metabolic demands. We used elemental microanalysis (SXRF) of single-celled plankton to determine their trace metal contents during a series of semi-Lagrangian drift studies in an HNLC upwelling region, the Costa Rica Dome (CRD). Cells from the surface mixed layer had lower Fe:S but higher Zn:S and Ni:S than those from the subsurface chlorophyll maximum at 22-30 m. Diatom Fe:S values were typically 3-fold higher than those in flagellated cells. The ratios of Zn:C in flagellates and diatoms were generally similar to each other, and to co-occurring mesozooplankton. Estimated Fe:C ratios in flagellates were lower than those in co-occurring mesozooplankton, sometimes by more than 3-fold. In contrast, Fe:C in diatoms was typically similar to that in zooplankton. RNA:DNA ratios in the CRD were low compared with other regions, and were related to total autotrophic biomass and weakly to the discrepancy between Zn:C in flagellated cells and mesozooplankton tissues. Mesozooplankton may have been affected by the trace element content of their food, even though trace metal limitation of phytoplankton was modest at best.

Phytoplankton production and taxon-specific growth rates in the Costa Rica Dome

During summer 2010, we investigated phytoplankton production and growth rates at 19 stations in the eastern tropical Pacific, where winds and strong opposing currents generate the Costa Rica Dome (CRD), an open-ocean upwelling feature. Primary production (¹⁴C-incorporation) and group-specific growth and net growth rates (two-treatment seawater dilution method) were estimated from samples incubated in situ at eight depths. Our cruise coincided with a mild El Niño event, and only weak upwelling was observed in the CRD. Nevertheless, the highest phytoplankton abundances were found near the dome center. However, mixed-layer growth rates were lowest in the dome center (∼0.5-0.9 day^-1), but higher on the edge of the dome (∼0.9-1.0 day^-1) and in adjacent coastal waters (0.9-1.3 day^-1). We found good agreement between independent methods to estimate growth rates. Mixed-layer growth rates of Prochlorococcus and Synechococcus were largely balanced by mortality, whereas eukaryotic phytoplankton showed positive net growth (∼0.5-0.6 day^-1), that is, growth available to support larger (mesozooplankton) consumer biomass. These are the first group-specific phytoplankton rate estimates in this region, and they demonstrate that integrated primary production is high, exceeding 1 g C m^-2 day^-1 on average, even during a period of reduced upwelling.

Biomass and composition of protistan grazers and heterotrophic bacteria in the Costa Rica Dome during summer 2010

We investigated biomass and composition of heterotrophic microbes in the Costa Rica Dome during June-July 2010 as part of a broader study of plankton trophic dynamics. Because picophytoplankton (<2 μm) are known to dominate in this unique upwelling region, we hypothesized tight biomass relationships between size-determined predator-prey pairs (i.e. picoplankton-nano-grazers, nanoplankton-micro-grazers) within the microbial community. Integrated biomass of heterotrophic bacteria ranged from 180 to 487 mg C m^-2 and was significantly correlated with total autotrophic carbon. Heterotrophic protist (H-protist) biomass ranged more narrowly from 488 to 545 mg C m^-2, and was comprised of 60% dinoflagellates, 30% other flagellates and 11% ciliates. Nano-sized (<20 μm) protists accounted for the majority (57%) of grazer biomass and were positively correlated with picoplankton, partially supporting our hypothesis, but nanoplankton and micro-grazers (>20 μm) were not significantly correlated. The relative constancy of H-protist biomass among locations despite clear changes in integrated autotrophic biomass, Chl a, and primary production suggests that mesozooplankton may exert a tight top-down control on micro-grazers. Biomass-specific consumption rates of phytoplankton by protistan grazers suggest an instantaneous growth rate of 0.52 day^-1 for H-protists, similar to the growth rate of phytoplankton and consistent with a trophically balanced ecosystem dominated by pico-nanoplankton interactions.

Vertical distribution of Eucalanoid copepods within the Costa Rica Dome area of the Eastern Tropical Pacific

A variety of ecological strategies for tolerance of low-oxygen conditions within the Costa Rica Dome (CRD) area of the Eastern Tropical Pacific are documented for the copepod family Eucalanidae. During the summer of 2010, we compared the ecological strategies used by the Eucalanidae inside and outside the central CRD region. We compared the vertical and horizontal distributions of five species, Eucalanus inermis, Subeucalanus subtenuis, Subeucalanus subcrassus, Subeucalanus pileatus and Pareucalanus attenuatus together with Rhincalanus species, in the epipelagic (upper 200 m) among four locations, which we grouped into a section roughly crossing the core CRD area (inside-outside core CRD). The coastal area outside the CRD supported the most diverse assemblage, whereas overall abundance of Eucalanidae in the central CRD was 2-fold greater than outside and dominated by E. inermis (>60%). Eucalanidae in the central CRD had a shallow depth distribution, closely associated with the shallow thermocline (10-20 m). There was no evidence of daily vertical migration in the central CRD, but E. inermis demonstrated vertical migration outside the CRD. The vertical abundance patterns of Eucalanidae in the CRD region reflect complex interactions between subtle physical-chemical differences and food resources.

Phytoplankton growth and microzooplankton grazing dynamics across vertical environmental gradients determined by transplant in situ dilution experiments

The Costa Rica Dome (CRD) represents a classic case of the bloom-forming capacity of small phytoplankton. Unlike other upwelling systems, autotrophic biomass in the CRD is dominated by picocyanobacteria and small eukaryotes that outcompete larger diatoms and reach extremely high biomass levels. We investigated responses of the subsurface phytoplankton community of the CRD to changes associated with vertical displacement of water masses, coupling in situ transplanted dilution experiments with flow cytometry and epifluorescence microscopy to assess group-specific dynamics. Growth rates of Synechococcus (SYN) and photosynthetic picoeukaryotes (PEUK) were positively correlated with light (R_{pearson_SYN} = 0.602 and R_{pearson_PEUK} = 0.588, P < 0.001). Growth rates of Prochlorococcus (PRO), likely affected by photoinhibition, were not light correlated (R_{pearson_PRO} = 0.101, P = 0.601). Overall, grazing and growth rates were closely coupled in all picophytoplankton groups (R_{spearman_PRO} = 0.572, R_{spearman_SYN} = 0.588, R_{spearman_PEUK} = 0.624), and net growth rates remained close to zero. Conversely, the abundance and biomass of larger phytoplankton, mainly diatoms, increased more than 10-fold in shallower transplant incubations indicating that, in addition to trace-metal chemistry, light also plays a significant role in controlling microphytoplankton populations in the CRD.

Net biogenic silica production and the contribution of diatoms to new production and organic matter export in the Costa Rica Dome ecosystem

We determined the net rate of biogenic silica (bSiO₂) production and estimated the diatom contribution to new production and organic matter export in the Costa Rica Dome during summer 2010. The shallow thermocline significantly reduces bSiO₂ dissolution rates below the mixed layer, leading to significant enhancement of bSiO₂ relative to organic matter (silicate-pump condition). This may explain why deep export of bSiO₂ in this region is elevated by an order of magnitude relative to comparable systems. Diatom carbon, relative to autotrophic carbon, was low (<3%); however, the contribution of diatoms to new production averaged 3 and 13% using independent approaches. The 4-old discrepancy between methods may be explained by a low average C:Si ratio (∼1.4) for the net produced diatom C relative to the net produced bSiO₂. We speculate that this low production ratio is not the result of reduced C, but may arise from a significant contribution of non-diatom silicifying organisms to bSiO₂ production. The contribution of diatoms to organic matter export was minor (5.7%). These results, and those of the broader project, suggest substantial food-web transformation of diatom organic matter in the euphotic zone, which creates enriched bSiO₂ relative to organic matter within the exported material.

Biological response of Costa Rica Dome phytoplankton to Light, Silicic acid and Trace metals

The Costa Rica Dome (CRD) is a unique open-ocean upwelling system, with picophytoplankton dominance of phytoplankton biomass and suppressed diatoms, yet paradoxically high export of biogenic silica. As a part of Flux and Zinc Experiments cruise in summer (June-July 2010), we conducted shipboard incubation experiments in the CRD to examine the potential roles of Si, Zn, Fe and light as regulating factors of phytoplankton biomass and community structure. Estimates of photosynthetic quantum yields revealed an extremely stressed phytoplankton population that responded positively to additions of silicic acid, iron and zinc and higher light conditions. Size-fractioned Chl a yielded the surprising result that picophytoplankton, as well as larger phytoplankton, responded most to treatments with added silicic acid incubated at high incident light (HL + Si). The combination of Si and HL also led to increases in cell sizes of picoplankton, notably in Synechococcus. Such a response, coupled with the recent discovery of significant intracellular accumulation of Si in some picophytoplankton, suggests that small phytoplankton could play a potentially important role in Si cycling in the CRD, which may help to explain its peculiar export characteristics.

The biological pump in the Costa Rica Dome: an open-ocean upwelling system with high new production and low export

The Costa Rica Dome is a picophytoplankton-dominated, open-ocean upwelling system in the Eastern Tropical Pacific that overlies the ocean's largest oxygen minimum zone. To investigate the efficiency of the biological pump in this unique area, we used shallow (90-150 m) drifting sediment traps and 234Th:238U deficiency measurements to determine export fluxes of carbon, nitrogen and phosphorus in sinking particles. Simultaneous measurements of nitrate uptake and shallow water nitrification allowed us to assess the equilibrium balance of new and export production over a monthly timescale. While f-ratios (new:total production) were reasonably high (0.36 ± 0.12, mean ± standard deviation), export efficiencies were considerably lower. Sediment traps suggested e-ratios (export/14C-primary production) at 90-100 m ranging from 0.053 to 0.067. ThE-ratios (234Th disequilibrium-derived export) ranged from 0.038 to 0.088. C:N and N:P stoichiometries of sinking material were both greater than canonical (Redfield) ratios or measured C:N of suspended particulates, and they increased with depth, suggesting that both nitrogen and phosphorus were preferentially remineralized from sinking particles. Our results are consistent with an ecosystem in which mesozooplankton play a major role in energy transfer to higher trophic levels but are relatively inefficient in mediating vertical carbon flux to depth, leading to an imbalance between new production and sinking flux.

Preferential depletion of zinc within Costa Rica upwelling dome creates conditions for zinc co-limitation of primary production

The Costa Rica Dome (CRD) is a wind-driven feature characterized by high primary production and an unusual cyanobacterial bloom in surface waters. It is not clear whether this bloom arises from top-down or bottom-up processes. Several studies have argued that trace metal geochemistry within the CRD contributes to the composition of the phytoplankton assemblages, since cyanobacteria and eukaryotic phytoplankton have different transition metal requirements. Here, we report that total dissolved zinc (Zn) is significantly depleted relative to phosphate (P) and silicate (Si) within the upper water column of the CRD compared with other oceanic systems, and this may create conditions favorable for cyanobacteria, which have lower Zn requirements than their eukaryotic competitors. Shipboard grow-out experiments revealed that while Si was a limiting factor under our experimental conditions, additions of Si and either iron (Fe) or Zn led to higher biomass than Si additions alone. The addition of Fe and Zn alone did not lead to significant enhancements. Our results suggest that the depletion of Zn relative to P in upwelled waters may create conditions in the near-surface waters that favor phytoplankton with low Zn requirements, including cyanobacteria.

Phytoplankton production and grazing balances in the Costa Rica Dome

We investigated phytoplankton production rates and grazing fates in the Costa Rica Dome (CRD) during summer 2010 based on dilution depth profiles analyzed by flow cytometry and pigments and mesozooplankton grazing assessed by gut fluorescence. Three community production estimates, from ¹⁴C uptake (1025 ± 113 mg C m^-2 day^-1) and from dilution experiments analyzed for total Chla (990 ± 106 mg C m^-2 day^-1) and flow cytometry populations (862 ± 71 mg C m^-2 day^-1), exceeded regional ship-based values by 2-3-fold. Picophytoplankton accounted for 56% of community biomass and 39% of production. Production profiles extended deeper for Prochlorococcus (PRO) and picoeukaryotes than for Synechococcus (SYN) and larger eukaryotes, but 93% of total production occurred above 40 m. Microzooplankton consumed all PRO and SYN growth and two-third of total production. Positive net growth of larger eukaryotes in the upper 40 m was balanced by independently measured consumption by mesozooplankton. Among larger eukaryotes, diatoms contributed ∼3% to production. On the basis of this analysis, the CRD region is characterized by high production and grazing turnover, comparable with or higher than estimates for the eastern equatorial Pacific. The region nonetheless displays characteristics atypical of high productivity, such as picophytoplankton dominance and suppressed diatom roles.

Mesozooplankton biomass and grazing in the Costa Rica Dome: amplifying variability through the plankton food web

We investigated standing stocks and grazing rates of mesozooplankton assemblages in the Costa Rica Dome (CRD), an open-ocean upwelling ecosystem in the eastern tropical Pacific. While phytoplankton biomass in the CRD is dominated by picophytoplankton (<2-µm cells) with especially high concentrations of Synechococcus spp., we found high mesozooplankton biomass (∼5 g dry weight m^-2) and grazing impact (12-50% integrated water column chlorophyll a), indicative of efficient food web transfer from primary producers to higher levels. In contrast to the relative uniformity in water-column chlorophyll a and mesozooplankton biomass, variability in herbivory was substantial, with lower rates in the central dome region and higher rates in areas offset from the dome center. While grazing rates were unrelated to total phytoplankton, correlations with cyanobacteria (negative) and biogenic SiO₂ production (positive) suggest that partitioning of primary production among phytoplankton sizes contributes to the variability observed in mesozooplankton metrics. We propose that advection of upwelled waters away from the dome center is accompanied by changes in mesozooplankton composition and grazing rates, reflecting small changes within the primary producers. Small changes within the phytoplankton community resulting in large changes in the mesozooplankton suggest that the variability in lower trophic level dynamics was effectively amplified through the food web.

Factors affecting Fe and Zn contents of mesozooplankton from the Costa Rica Dome

Mineral limitation of mesozooplankton production is possible in waters with low trace metal availability. As a step toward estimating mesozooplankton Fe and Zn requirements under such conditions, we measured tissue concentrations of major and trace nutrient elements within size-fractioned zooplankton samples collected in and around the Costa Rica Upwelling Dome, a region where phytoplankton growth may be co-limited by Zn and Fe. The geometric mean C, N, P contents were 27, 5.6 and 0.21 mmol gdw^-1, respectively. The values for Fe and Zn were 1230 and 498 nmol gdw^-1, respectively, which are low compared with previous measurements. Migrant zooplankton caused C and P contents of the 2-5 mm fraction to increase at night relative to the day while the Fe and Zn contents decreased. Fe content increased with size while Zn content decreased with size. Fe content was strongly correlated to concentrations of two lithogenic tracers, Al and Ti. We estimate minimum Fe:C ratios in large migrant and resident mixed layer zooplankton to be 15 and 60 µmol mol^-1, respectively. The ratio of Zn:C ranged from 11 µmol mol^-1 for the 0.2-0.5 mm size fraction to 33 µmol mol^-1 for the 2-5 mm size fraction.

Diazotroph community structure in the deep oxygen minimum zone of the Costa Rica Dome

Oxygen minimum zones (OMZs), characterized by depleted dissolved oxygen concentration in the intermediate depth of the water column, are predicted to expand under the influence of global warming. Recent studies in the Eastern Tropical South Pacific Ocean and Arabian Sea have reported that heterotrophic nitrogen fixation is active in the OMZs. In this study, we investigated the community structure of diazotrophs in the OMZ of the Costa Rica Dome (CRD) upwelling region in the Eastern Tropical North Pacific Ocean, using 454-pyrosequencing of nifH gene amplicons. Comparing diazotroph assemblages in different depth strata of the OMZ (200-1000 m in depth), we found a unique diazotroph community in the OMZ core, which was mainly dominated by methanotroph-like diazotrophs, suggesting a potential coupling of nitrogen cycle and methane assimilation. In addition, some OTUs revealed in this study, especially those belonging to the large sub-cluster Vibrio diazotrophicus, were reported to be abundant and expressing the nifH gene in other OMZs. Our results suggest that the unique hydrographic conditions in OMZs may support similar assemblages of diazotrophs, and heterotrophic nitrogen fixation could also be occurring in our studied region. Our study provides the first insight into the composition and distribution of putative diazotrophs in the CRD OMZ.

Patterns of microbial community biomass, composition and HPLC diagnostic pigments in the Costa Rica upwelling dome

We investigated biomass, size-structure, composition, depth distributions and spatial variability of the phytoplankton community in the Costa Rica Dome (CRD) in June-July 2010. Euphotic zone profiles were sampled daily during Lagrangian experiments in and out of the dome region, and the community was analyzed using a combination of digital epifluorescence microscopy, flow cytometry and HPLC pigments. The mean depth-integrated biomass of phytoplankton ranged 2-fold, from 1089 to 1858 mg C m^-2 (mean ± SE = 1378 ± 112 mg C m^-2), among 4 water parcels tracked for 4 days. Corresponding mean (±SE) integrated values for total chlorophyll a (Chl a) and the ratio of autotrophic carbon to Chl a were 24.1 ± 1.5 mg Chl a m^-2 and 57.5 ± 3.4, respectively. Absolute and relative contributions of picophytoplankton (∼60%), Synechococcus (>33%) and Prochlorococcus (17%) to phytoplankton community biomass were highest in the central dome region, while >20 µm phytoplankton accounted for ≤10%, and diatoms <2%, of biomass in all areas. Nonetheless, autotrophic flagellates, dominated by dinoflagellates, exceeded biomass contributions of Synechococcus at all locations. Order-of-magnitude discrepancies in the relative contributions of diatoms (overestimated) and dinoflagellates (underestimated) based on diagnostic pigments relative to microscopy highlight potential significant biases associated with making community inferences from pigments.