Phytoplankton Production Rates Research Articles

The bottom mixed layer depth as an indicator of subsurface Chlorophyll a distribution

Abstract. Primary production dynamics are strongly associated with vertical density profiles in shelf waters. Variations in the vertical structure of the pycnocline in stratified shelf waters are likely to affect nutrient fluxes and hence the vertical distribution and production rate of phytoplankton. To understand the effects of physical changes on primary production, identifying the linkage between water column density and Chlorophyll a (Chl a) profiles is essential. Here, the vertical distributions of density features describing three different portions of the pycnocline (the top, centre, and bottom) were compared to the vertical distribution of Chl a to provide auxiliary variables to estimate Chl a in shelf waters. The proximity of density features with deep Chl a maximum (DCM) was tested using the Spearman correlation, linear regression, and a major axis regression over 15 years in a shelf sea region (the northern North Sea) that exhibits stratified water columns. Out of 1237 observations, 78 % reported DCM above the bottom mixed layer depth (BMLD: depth between the bottom of the pycnocline and the mixed layer underneath) with an average distance of 2.74 ± 5.21 m from each other. BMLD acts as a vertical boundary above which subsurface Chl a maxima are mostly found in shelf seas (depth ≤ 115 m). Overall, DCMs were correlated with the halfway pycnocline depth (HPD) (ρS = 0.56) which, combined with BMLD, were better predictors of the locations of DCMs than surface mixed layer indicators and the maximum squared buoyancy frequency. These results suggest a significant contribution of deep mixing processes in defining the vertical distribution of subsurface production in stratified waters and indicate BMLD as a potential indicator of the Chl a spatiotemporal variability in shelf seas. An analytical approach integrating the threshold and the maximum angle method is proposed to extrapolate BMLD, the surface mixed layer, and DCM from in situ vertical samples.

Seasonal Dynamics of Primary Production in the Southeastern Bering Sea Assessed Using Continuous Temporal and Vertical Dissolved Oxygen and Chlorophyll‐a Measurements

AbstractThe Bering Sea and other high‐latitude systems are experiencing unprecedented changes related to climate warming and the consequent loss of sea ice. Understanding how such changes influence primary production is a pressing question. Here, we quantify primary production rates in the southeastern Bering Sea over 4 years (2016–2019) at daily to weekly time resolution. A moored high‐resolution Profiling Crawler was used in combination with in situ sampling to collect physical and biological data in the ocean's upper 50 m. We used dissolved oxygen (O2) data to estimate gross and net primary production (NPP), and chlorophyll‐a (Chl‐a), temperature, and irradiance data to model NPP rates. We then assessed seasonal variation in phytoplankton production rates, phytoplankton community growth rates () and explored to what extent summer phytoplankton community growth rates may be influenced by nitrogen limitation. Our analyses revealed that the majority of gross primary production (GPP) and net community production occurs in association with the spring phytoplankton bloom. After the bloom, the water column generally experienced low GPP and net biological carbon consumption. Phytoplankton growth rates were commonly suppressed in late summer due to apparent nitrogen depletion. Using high temporal resolution vertically resolved measurements of O2 and Chl‐a, our analyses provides important insight into how biogeochemical cycles, phytoplankton community growth rates, and carbon available for export vary seasonally in the southeastern Bering Sea.

Bluefin Larvae in Oligotrophic Ocean Foodwebs, investigations of nutrients to zooplankton: overview of the BLOOFINZ-Gulf of Mexico program

Abstract Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds in the North Atlantic to spawn in oligotrophic waters of the Gulf of Mexico (GoM). Stock recruitment is strongly affected by interannual variability in the physical features associated with ABT larvae, but the nutrient sources and food-web structure of preferred habitat, the edges of anticyclonic loop eddies, are unknown. Here, we describe the goals, physical context, design and major findings of an end-to-end process study conducted during peak ABT spawning in May 2017 and 2018. Mesoscale features in the oceanic GoM were surveyed for larvae, and five multi-day Lagrangian experiments measured hydrography and nutrients; plankton biomass and composition from bacteria to zooplankton and fish larvae; phytoplankton nutrient uptake, productivity and taxon-specific growth rates; micro- and mesozooplankton grazing; particle export; and ABT larval feeding and growth rates. We provide a general introduction to the BLOOFINZ-GoM project (Bluefin tuna Larvae in Oligotrophic Ocean Foodwebs, Investigation of Nitrogen to Zooplankton) and highlight the finding, based on backtracking of experimental waters to their positions weeks earlier, that lateral transport from the continental slope region may be more of a key determinant of available habitat utilized by larvae than eddy edges per se.

Phytoplankton production and consumption rate in the warm and cold seasons in the coastal ecosystem of Matsu Island

Phytoplankton production and consumption rate in the warm and cold seasons in the coastal ecosystem of Matsu Island

Response of oligotrophic coastal microbial populations in the SE Mediterranean Sea to crude oil pollution; lessons from mesocosm studies

Anthropogenically-induced oil spills release large amounts of organic pollutants into the marine environment. To date, little is known about the response of microbial populations (biomass, activity and diversity) to crude oil pollution in Low Nutrients Low Chlorophyll and warm systems. Here, we investigated the daily dynamics of phytoplankton and heterotrophic bacteria in response to an oil spill (500 μm thick layer) in the coastal waters of the SE Mediterranean Sea (SEMS), using mesocosms during winter and summer. Crude oil addition caused a marked decrease in phytoplankton biomass (40–76%) and production rates (22–96%), whereas heterotrophic bacterial abundance and production increased (4–68% and 17–165%, respectively). Concurrently, amplicon sequencing of the 16S rRNA gene revealed that oil-degrading bacteria became abundant 48–96 h post-oil addition, while the cosmopolitan Synechococcus and SAR11 lineages were significantly reduced (by 78–98% and 59–98%, respectively). Fertilization with inorganic nutrients (NO3 and PO4) reduced the deleterious effects of the oil, resulting in a less distinct reduction in phytoplankton biomass/abundance. Our results highlight the potential of intrinsic microbial communities to degrade oil-derived pollutants in oligotrophic coastal waters.

Temperature acclimation alters phytoplankton growth and production rates

AbstractTemperature is a major driver of phytoplankton growth and physiology, but despite decades of study on temperature effects, the influence of temperature fluctuations on the growth acclimation of marine phytoplankton is largely unknown. To address this knowledge gap, we subjected a coastal phytoplankton species, Heterosigma akashiwo, to ecologically relevant temperature shifts of 2–3°C, cumulatively totaling 3–16°C across a range from 6°C to 31°C over a 3‐week period. Using a symmetric design, we show time dependent differences between growth rates and that these changes were related to the magnitude of the temperature shift, but not the direction. Cell size scaled inversely with temperature at a rate of −1.9 to −3.3%°C−1 at all except the highest temperature treatments > 25°C. Intraspecific variability in growth rates increased exponentially with cumulative thermal shifts, suggesting thermal variability may be a driver of intraspecific variation. The observed acclimation effects on phytoplankton growth rates suggest that ignoring acclimation effects could systematically under or overestimate temperature‐dependent primary production. Empirical results, contextualized with in situ coastal ocean temperature record, demonstrated that daily primary production could differ from current model assumptions utilizing acclimated rates by −33% to +36%. If broadly applicable to diverse phytoplankton species, these results have ramifications for predicting the ecology and production of phytoplankton in present day dynamic ecosystems and in future climate scenarios where thermal variability is expected to increase.

Increase in phytoplankton production and consumption in response to a typhoon passing through the southern East China Sea

The northwestern Pacific Ocean and East China Sea (ECS) are frequently affected by typhoons during the summer period. To identify changes in production and consumption rates of phytoplankton (<20 and 20~200 µm) affected by Typhoon Maria (9-10 July 2018), 2 sampling periods were compared: a pre-typhoon cruise (4-9 July 2018) and a post-typhoon cruise (13-17 July 2018). Results showed that <20 and 20~200 µm phytoplankton production was 14- and 1.8-fold higher, respectively, in the post-typhoon period compared to the pre-typhoon. Accompanying this post-typhoon shift from pico- and nanophytoplankton to microphytoplankton, the consumption rate of microzooplankton on <20 µm phytoplankton was 11.6 mgC m-3 d-1 in the pre-typhoon period, and increased to 203.3 mgC m-3 d-1 after the typhoon. Furthermore, the ingestion fluxes were 4.9 and 8.0 mgC m-3 d-1 through mesozooplankton grazing on microzooplankton (ciliates and dinoflagellates) pre- and post-typhoon, respectively. Our findings show that organisms from the microbial food web (i.e. ciliates and dinoflagellates) may control the phytoplankton assemblage during the post-typhoon period of increased productivity, and thus represent an important trophic link between the classical and microbial food webs in the southern ECS.

Synoptic Spatio-Temporal Variability of the Photosynthetic Productivity of Microphytobenthos and Phytoplankton in a Tidal Estuary

Tidal estuaries are regarded as highly important ecosystems, mostly due to their high primary productivity and associated role as carbon sinks. In these ecosystems, primary productivity is mainly due to the photosynthetic carbon fixation by phytoplankton and microphytobenthos. The productivity of the two communities has been mostly studied separately, and directly comparable estimates of their carbon fixation rates in the same estuary are relatively scarce. The present study aimed to characterize the spatio-temporal variability of the productivity of phytoplankton and microphytobenthos in a tidal estuary, the Ria de Aveiro (Portugal), and to estimate the annual ecosystem-level budget for carbon fixation by the two communities. Productivity rates were determined based on synoptic in situ measurements of absolute rates of electron transport rate of photosystem II, using PAM fluorometry. Chlorophyll fluorescence indices were accompanied by measurements of salinity, temperature, water turbidity, solar irradiance, and planktonic and benthic microalgal biomass. Measurements were carried out hourly, along four spring-neap tidal cycles distributed along one year, on three sites of the estuary. The most pronounced trends in the spatio-temporal variability of the photophysiology and productivity of the two communities were the following: (i) maximum biomass and productivity were reached later for microphytobenthos (summer-autumn) than for phytoplankton (spring-summer); (ii) the absorption cross-section of PSII was generally higher for phytoplankton; (iii) the two groups showed a similar photoacclimation state, but microphytobenthos appeared as high light-acclimated when compared to phytoplankton. Biomass-specific productivity was on average higher for phytoplankton than for microphytobenthos, averaging 68.0 and 19.1 mg C mg Chl a-1 d-1, respectively. However, areal depth-integrated production rates were generally higher for the microphytobenthos than for the phytoplankton, averaging 264.5 and 140.0 mg C m-2 d-1, respectively. On an annual basis, phytoplankton productivity averaged 49.9 g C m-2 yr-1 while the productivity of microphytobenthos averaged 105.2 g C m-2 yr-1. When upscaling to the whole estuary, annual primary production rates of phytoplankton and microphytobenthos reached 4894.3 and 7534.0 t C yr-1, respectively, representing 39.4% and 60.6% of the combined total of 12428.3 t C yr-1 determined for the two communities in the Ria de Aveiro.

Marine system dynamical response to a changing climate in frame of power law, exponential decay, and Mittag‐Leffler kernel

The increase of sea surface temperature in ocean changes the photosynthetic production rate of phytoplankton. Therefore, it is crucial to understand the relation between temperature and phytoplanktons photosynthesis to deal the extinction caused by excessive increase in temperature. It is worth observing that temperature is one of the most principal limiting factors for phytoplanktons production due to photosynthetic enzymes work at their optimum temperature levels. In this study, the fractional oxygen‐phytoplankton‐zooplankton model is considered by singular and nonsingular fractional operators within Caputo, Caputo‐Fabrizio, and Atangana‐Baleanu in Caputo sense. The rate of oxygen production is considered by a function of temperature account for the sea surface warming. At first, the temperature function is constant and then it starts to increase, after a certain time of increase, before the oxygen depletion begins, the temperature is set to a higher secure value. With this temperature function choice, detailed numerical simulations are carried out to provide details of the internal structure of the system. We observe that the species are more sustainable in Caputo model than its corresponding integer‐order model.

Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

AbstractThe Subantarctic zone is one of the largest High‐Nutrient Low‐Chlorophyll zones of the Southern Ocean. Despite widespread iron limitation, phytoplankton accumulation (chlorophyll a (chla) > 0.3 mg m−3) often occurs near islands and bathymetric features such as on the Campbell Plateau, southeast of New Zealand. To investigate the processes responsible for localized increases in chla commonly observed by satellites, we characterized phytoplankton biomass structure, production, and microzooplankton grazing on Campbell Plateau and surrounding waters in austral autumn (March 2017). Chla on the plateau tended to be higher, more variable (0.52 ± 0.38 mg chla m−3, mean ± standard deviation), and characterized by larger phytoplankton forms (22 ± 27%chla > 20 μm) than surrounding waters (0.29 ± 0.12 mg chla m−3, 5 ± 2%). The increased contribution of diatoms, together with higher photosystem II maximum photochemical efficiency (Fv/Fm = 0.45 ± 0.05) and lower effective absorption cross‐section (σPSII = 774 ± 90 Å RCII−1) on the plateau, suggests an alleviation of iron stress relative to surrounding waters (Fv/Fm = 0.37 ± 0.04, σPSII = 974 ± 89 Å RCII−1). Phytoplankton growth (μ0 = 0.42 ± 0.20 day−1) and production rates (6.1 ± 3.2 mg C m−3 day−1) were also higher compared to surrounding waters (0.27 ± 0.04 day−1, 3.5 ± 1.9 mg C m−3 day−1). While microzooplankton grazing (g = 0.28 ± 0.18 day−1) balanced phytoplankton growth off the plateau (g:μ0 = 1.13 ± 0.18), the imbalance observed on Campbell Plateau (g = 0.25 ± 0.25 day−1) allowed a substantial proportion of primary production to escape microzooplankton grazing control (g:μ0 = 0.48 ± 0.31). Overall, the degree of coupling tended to decrease with the depth of the mixed layer (R2 > 0.6, p < 0.001). We hypothesize that the entrainment of deeper water into the mixed layer regulates the onset and fate of the autumn bloom by altering nutrient supply and microzooplankton grazing pressure.

Drivers of Microbial Carbon Fluxes Variability in Two Oligotrophic Mediterranean Coastal Systems

The carbon fluxes between phytoplankton and heterotrophic bacterioplankton were studied in two coastal oligotrophic sites in the NW Mediterranean. Phytoplankton and bacterial production rates were measured under natural conditions using different methods. In the Bay of Villefranche, the temporal variability revealed net heterotrophy in July-October and net autotrophy in December-March. The spatial variability was studied in the Bay of Palma, showing net autotrophic areas in the west and heterotrophic areas in the east. On average bacterial respiration, represented 62% of the total community respiration. Bacterial growth efficiency (BGE) values were significantly higher in autotrophic conditions than in heterotrophic ones. During autotrophic periods, dissolved primary production (DPP) was enough to sustained bacterial metabolism, although it showed a positive correlation with organic carbon stock (DOC). Under heterotrophic conditions, DPP did not sustain bacterial metabolism but bacterial respiration correlated with DPP and bacterial production with DOC. Temperature affected positively, DOC, BGE, bacterial respiration and production when the trophic status was autotrophic. To summarize, the response of bacterial metabolism to temperature and carbon sources depends on the trophic status within these oligotrophic coastal systems.

Patterns in phytoplankton and benthic production on the shallow continental shelf in the northeastern Gulf of Mexico

Shallow continental shelves support productive pelagic and benthic communities. This study examined primary productivity at a shallow shelf region in the northeastern Gulf of Mexico focusing on the effect of light on water column and benthic productivity at water depths between 12 and 17 m. Measurements were made between November 2015 and September 2016. Dissolved oxygen fluxes were measured using benthic chambers with four different light levels and used to calculate gross primary production and respiration. Phytoplankton productivity was measured using 14C-uptake incubations in a laboratory photosynthetron. Organic matter production by benthic microalgae is substantial in this region of northeastern Gulf of Mexico with daily production rates ranging from 0.1 to 0.8 g C m−2 d−1 in this study. Maximum rates of phytoplankton production up to 2.7 g C m−2 d−1 occurred in spring. This peak productivity followed wind conditions favorable to upwelling and occurred when bottom water NO3− concentrations were 11 times greater than on any other sample date during the study. At these shallow depths, benthic microalgae made a significant contribution to total shelf production, averaging about 14% of total production. These results helped characterize benthic and water column production rates prior to planned habitat alterations caused by placement of numerous artificial reef structures in the region.

Winter‐mixing preconditioning of the spring phytoplankton bloom in the Bay of Biscay

AbstractThe spring phytoplankton bloom plays a key role in the dynamics of temperate and polar seas. Nevertheless, the mechanisms and processes behind these blooms remain a subject of considerable debate. We analyzed the influence of deep mixing during winter on the spring phytoplankton bloom in the Cantabrian Sea (southern Bay of Biscay). To this end, we combined long‐term physical and biogeochemical in situ data (1993–2012) and satellite observations (1997–2012). Deeper winter mixing led to higher nitrate and chlorophyll concentrations through the water column during the spring bloom. However, this effect was modified by short‐term variability in near‐surface stratification in spring. Winter‐mixing preconditioning also influenced different spring bloom metrics: deeper and later mixing in winter was followed by later blooms with a larger peak. In these enhanced blooms, nitrate was taken up at faster rates, indicating higher rates of phytoplankton production. Winters with weaker mixing (that led to weaker spring blooms) were associated with warmer surface temperatures. This relationship suggests that the multi‐decadal trend toward warmer surface temperatures in the Bay of Biscay may promote a decrease in the magnitude of the spring bloom, which could impact upper trophic levels and also deep carbon export in the future.

Novel incubation‐free approaches to determine phytoplankton net primary productivity, growth, and biomass based on flow cytometry and quantification of ATP and NAD(H)

AbstractRecent advances in sorting flow cytometry (SFCM) and sensitive biochemical techniques have opened the door for a variety of new incubation‐free measurements to assess cell physiology. We adapted highly sensitive bioluminescent methodologies to measure cell NAD(H) and adenosine triphosphate (ATP) contents as predictors of phytoplankton primary production and growth rate. We first applied these methods to algae in taxonomically distinct groups: the chlorophyte Dunaliella tertiolecta, the diatom Thalassiosira pseudonana and a model cyanobacterium Synechococcus sp. WH8102. We found that NAD(H) content [NAD(H)phyto] increased significantly with increasing net primary productivity (NPP) and growth rate. In contrast, and in agreement with previous studies, phytoplankton ATP content (ATPphyto) was highly correlated with carbon biomass. Furthermore, ATPphyto was not correlated with NPP or growth rate. As a field test of the method, we then measured NAD(H)phyto in phytoplankton samples collected by SFCM in the North Pacific. These measurements yielded an estimate of NPP closely matching that from the traditional incubation‐dependent 14C uptake technique. The methods described here enable low‐biomass, incubation‐free estimates of phytoplankton NPP, growth rate, and biomass.

Small phytoplankton contribution to the standing stocks and the total primary production in the Amundsen Sea

Abstract. Small phytoplankton are anticipated to be more important in a recently warming and freshening ocean condition. However, little information on the contribution of small phytoplankton to overall phytoplankton production is currently available in the Amundsen Sea. To determine the contributions of small phytoplankton to total biomass and primary production, carbon and nitrogen uptake rates of total and small phytoplankton were obtained from 12 productivity stations in the Amundsen Sea. The daily carbon uptake rates of total phytoplankton averaged in this study were 0.42 g C m−2 d−1 (SD = ± 0.30 g C m−2 d−1) and 0.84 g C m−2 d−1 (SD = ± 0.18 g C m−2 d−1) for non-polynya and polynya regions, respectively, whereas the daily total nitrogen (nitrate and ammonium) uptake rates were 0.12 g N m−2 d−1 (SD = ± 0.09 g N m−2 d−1) and 0.21 g N m−2 d−1 (SD = ± 0.11 g N m−2 d−1), respectively, for non-polynya and polynya regions, all of which were within the ranges reported previously. Small phytoplankton contributed 26.9 and 27.7 % to the total carbon and nitrogen uptake rates of phytoplankton in this study, respectively, which were relatively higher than the chlorophyll a contribution (19.4 %) of small phytoplankton. For a comparison of different regions, the contributions for chlorophyll a concentration and primary production of small phytoplankton averaged from all the non-polynya stations were 42.4 and 50.8 %, which were significantly higher than those (7.9 and 14.9 %, respectively) in the polynya region. A strong negative correlation (r2 = 0. 790, p<0. 05) was found between the contributions of small phytoplankton and the total daily primary production of phytoplankton in this study. This finding implies that daily primary production decreases as small phytoplankton contribution increases, which is mainly due to the lower carbon uptake rate of small phytoplankton than large phytoplankton.

Decadal phytoplankton dynamics in response to episodic climatic disturbances in a subtropical deep freshwater ecosystem

Information of the decadal timescale effects of episodic climatic disturbances (i.e., typhoons) on phytoplankton in freshwater ecosystems have received less attention and fewer seasonal evaluations partly due to the lack of long-term time-series monitoring data in typhoon prevailing areas. Through field observations of a total 36 typhoon cases in a subtropical deep freshwater ecosystem in the period of 2005–2014, we quantified phytoplankton biomass, production and growth rate in response to meteorological and hydrological changes in the weeks before, during and after typhoons between summer and autumn, and also investigated the effects of typhoon characteristics on the aforementioned phytoplankton responses. The results showed that phytoplankton exposed to typhoon disturbances generally exhibited an increasing trend over the weeks before, during and after typhoons in summer but varied in autumn. The correlations and multivariate regressions showed different contributions of meteorological and hydrological variables to individual phytoplankton responses before, during and after typhoons between seasons. The post-typhoon weeks (i.e., within two weeks after a typhoon had passed) were especially important for the timeline of phytoplankton increases and with a detectable seasonal variation that the chlorophyll a concentration significantly increased in autumn whereas both primary production and growth rate were associated with significant changes in summer. Additionally, phytoplankton responses during the post-typhoon weeks were significantly different between discrete or continuous types of typhoon events. Our work illustrated the fact that typhoons did influence phytoplankton responses in the subtropical deep freshwater ecosystem and typhoon passages in summer and autumn affected the phytoplankton dynamics differently. Nevertheless, sustained and systematic monitoring in order to advance our understanding of the role of typhoons between seasons in the modulation of phytoplankton productivity and functioning is required because such episodic climatic disturbances are projected to have intense magnitude and inconsistent frequency under 21st century climate change.

Recovery of photosynthesis parameters fromin situprofiles of phytoplankton production

We examine a model of the rate of phytoplankton production in the ocean and its dependence on depth. The model is analysed as a function of photosynthesis parameters and it is shown that: (i) production profiles with depth are determined uniquely by the parameter values ; (ii) daily water column production is not uniquely determined by the parameter values ; (iii) a unique combination of parameters exists for which the model best fits a measured production profile. An inverse procedure is developed to recover photosynthesis parameters from measured profiles of primary production, and its performance tested by application to profiles of primary production collected at the Hawaii Ocean Time Series. For each profile tested, the method is successful in recovery of the photosynthesis parameters. The method can be applied to the estimation of photosynthesis parameters from data on in situ production profiles, which have been collected globally for more than half a century, thereby augmenting the world archive of these parameters for application in ecosystem modelling and estimation of primary production from remotely sensed data.

The Temporal Dynamics of Coastal Phytoplankton and Bacterioplankton in the Eastern Mediterranean Sea.

This study considers variability in phytoplankton and heterotrophic bacterial abundances and production rates, in one of the most oligotrophic marine regions in the world–the Levantine Basin. The temporal dynamics of these planktonic groups were studied in the coastal waters of the southeastern Mediterranean Sea approximately every two weeks for a total of two years. Heterotrophic bacteria were abundant mostly during late summer and midwinter, and were positively correlated with bacterial production and with N2 fixation. Based on size fractionating, picophytoplankton was abundant during the summer, whereas nano-microphytoplankton predominated during the winter and early spring, which were also evident in the size-fractionated primary production rates. Autotrophic abundance and production correlated negatively with temperature, but did not correlate with inorganic nutrients. Furthermore, a comparison of our results with results from the open Levantine Basin demonstrates that autotrophic and heterotrophic production, as well as N2 fixation rates, are considerably higher in the coastal habitat than in the open sea, while nutrient levels or cell abundance are not different. These findings have important ecological implications for food web dynamics and for biological carbon sequestration in this understudied region.

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.

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.