Depth-integrated Primary Production Research Articles

Does increased springtime solar radiation also increase primary production?

AbstractWe tested the hypothesis that long-term brightening of solar radiation and changes in the underwater light climate would increase the primary production of phytoplankton in the springtime in Lake Kasumigaura, Japan. Global solar radiation fluctuates on a decadal scale, and the solar radiation in May has been increasing since the 1990s in Japan. We developed a model of depth-integrated primary production (gC m−2 h−1) based on long-term monitoring data. The model based on photosynthesis-irradiance curve clarified that primary production increased by 13.2%, from 0.093 gC m−2 h−1 in 1992 to 0.105 gC m−2 h−1 in 2019, because of the combined effects of solar radiation and water temperature increases. Sensitivity analysis of the model showed that primary production was most sensitive to water quality parameters, i.e. the light attenuation coefficient and nutrient effects on Pmax. Nutrient effects were significantly correlated with the particulate organic carbon to nitrogen ratio, indicating that nitrogen shortage in phytoplankton cells may affect primary production. These results suggest that long-term springtime increases in solar radiation and water temperature increase primary production, but that water quality also controls primary production.

Remote estimation of phytoplankton primary production in clear to turbid waters by integrating a semi-analytical model with a machine learning algorithm

Remote estimation of phytoplankton primary production has long been recognized as an important method for investigating the responses of aquatic ecosystems to global climate change. The theory-based primary production model (TPM), one of the earlier proposed models, is potentially applicable to a variety of water bodies because of its semi-analytical nature. Its accuracy is highly dependent on whether the photophysiological response of phytoplankton is adequately parameterized, specifically the assimilation number (PmaxB) and the light saturation parameter (Ek). The remote assignment of PmaxB and Ek is acknowledged to be a challenging task, and the limited progress has impeded extensive use of the TPM. In this study, we proposed a machine learning algorithm, the enhanced random forest regression (ERFR), to retrieve PmaxB and Ek from satellite observations. The ERFR were then integrated with the TPM (together termed as TPMERFR) to estimate daily depth-integrated primary production (IPP) in clear to turbid waters. The ERFR was trained and validated using in situ datasets from a broad range of trophic and biogeographic conditions, covering oceanic, coastal, and inland water bodies. Evaluations with independent in situ data and matchup data showed that the ERFR outperformed conventional empirical and semi-analytical algorithms, and it could better capture the variability of PmaxB and Ek than look-up-table methods. The root mean square difference (RMSD) of the satellite-based IPP estimates from the TPMERFR remained within 0.27. In contrast, the benchmark models generally yielded IPP estimates with RMSDs of 0.27–0.62. The TPMERFR was then implemented to climatological satellite products (2010–2019) to reassess global IPP. Reasonable spatial distributions of IPP were preliminarily demonstrated, especially in polar, coastal, and inland waters. These results indicated the potential utility of the TPMERFR to generate seamless IPP distributions in clear to turbid waters.

Improved Perceptron of Subsurface Chlorophyll Maxima by a Deep Neural Network: A Case Study with BGC-Argo Float Data in the Northwestern Pacific Ocean

Subsurface chlorophyll maxima (SCMs), commonly occurring beneath the surface mixed layer in coastal seas and open oceans, account for main changes in depth-integrated primary production and hence significantly contribute to the global carbon cycle. To fill the gap of previous methods (in situ measurement, remote sensing, and the extrapolating function based on surface-ocean data) for obtaining SCM characteristics (intensity, depth, and thickness), we developed an improved deep neural network (IDNN) model using a Gaussian radial basis activation function to retrieve the vertical profile of chlorophyll a concentration (Chl a) and associated SCM characteristics from surface-ocean data. The annually averaged SCM depth was further incorporated into the bias term and the Gaussian activation function to improve the estimation accuracy of the IDNN model. Based on the Biogeochemical-Argo (BGC-Argo) data acquired for three regions in the northwestern Pacific Ocean, vertical Chl a profiles produced by our improved DNN model using sea surface Chl a and sea surface temperature (SST) were in good agreement with the observations, especially in regions with low surface Chl a. Compared to other neural-network-based models with one hidden layer and a sigmoid activation function, the IDNN model retrieved vertical Chl a profiles well in more eutrophic subpolar regions. Furthermore, the application of the IDNN model to infer vertical Chl a profiles from remote-sensing information was validated in the northwestern Pacific Ocean.

Phytoplankton primary production: 14C-in situ and 14C-incubator methods compared

Abstract We estimated phytoplankton depth-integrated primary production rates at an open sea station in the northern Baltic Proper in 2004–2009 by parallel in situ and ship-board incubations in an “ICES incubator”. Observations were strongly linearly related (r2 = 0.67, p < 0.00001) and produced similar seasonal signals, but incubator estimates were significantly lower in spring (∼5–20%, March–May) and summer (∼30%, June–August), but not in autumn (September–October). Incubator PBmax was significantly lower in all seasons (∼10, 23, and 14% in spring, summer, and autumn), and the initial slope of the PB/E-curve (αB) was two-thirds of in-situ αB in spring and autumn, and half in summer, but not significantly different in autumn, and had an order of magnitude lower standard deviation. Parallel incubations of mixed 0–10 m hose samples at discrete depths in situ and in the incubator gave similar differences in PBmax and αB. This strongly suggests that the main factor causing the lower photosynthesis rates in the incubator was light quality, since all other potential sources of variability were similar. However, we cannot rule out that the lower incubator PBmax was partly due to insufficient light compared to in situ, particularly in summer. Our results suggest that a major challenge to the harmonization of 14C in situ, and artificial light and deck incubator measurements, is to simulate spectral composition changes with depth or, alternatively, use phytoplankton action spectra or constructed action spectra that are less complicated and time-consuming to establish (Kyewalyanga et al., 1997).

Seasonal-to-decadal scale variability in primary production and particulate matter export at Station ALOHA

Station ALOHA (A Long-term Oligotrophic Habitat Assessment) was established in the North Pacific Subtropical Gyre (22°45′N, 158°W) as an oligotrophic ocean benchmark to improve our understanding of processes that govern the fluxes of carbon (C) into and from the surface ocean. At approximately monthly intervals, measurements of the primary production of particulate C (PC) using the 14C method, and the export of PC and particulate nitrogen (PN) using surface-tethered sediment traps deployed at 150 m have been made along with a host of complementary physical, biological, and biogeochemical measurements. Euphotic zone depth-integrated (0–200 m) primary production ranged from 220.2 (standard deviation, SD, 10.8) mg C m−2 d−1 in Feb 2018 to 1136.5 (SD = 17.1) mg C m−2 d−1 in Jun 2000, with a 30-yr (1989–2018) mean of 536.8 (SD = 135.0) mg C m−2 d−1 (n = 271). Although the monthly primary production climatology was fairly well constrained, we observed substantial sub-decadal variability and a significant 0–125 m depth-integrated increasing trend of 4.0 (p < 0.01; 95% confidence interval, CI, 2.1–5.9) (mg C m−2 d−1) yr−1 since 1989, displaying a large relative increase of 37% (CI = 18–55%) in the lower portion (75–125 m) of the euphotic zone. Chlorophyll (Chl) a and suspended PC and PN concentrations also displayed significant (p < 0.01) increases in the 75–125 m region of the euphotic zone. PC export at 150 m exhibited both short-term (monthly) and longer-scale variability with a 30-yr mean of 27.9 (SD = 9.7, n = 265) mg C m−2 d−1. PC and PN export exhibited extended, multi-year periods of significantly lower or higher values compared to the 30-yr mean. These multi-year periods of anomalously low and high particle export, in the absence of contemporaneous variations in primary production, probably reflect periodic changes in remineralization efficiencies. The PC export ratio (e-ratio; PC export at 150 m ÷ 0–150 m depth-integrated 14C-based primary production) was low, with a 30-yr mean of 0.054 (SD = 0.021, n = 248), and exhibited a significant (p < 0.01) long-term decreasing trend over the 30-yr observation period. The 30-yr long-term increases in primary production (~37%), Chl a, and suspended PC and PN concentrations (~17%, 8%, and 8%, respectively) in the 75–125 m portion of the water column are hypothesized to result from an enhanced supply of nutrients to the lower portion of the water column over the past three decades.

Seasonal and Spatial Variability of Primary Production in the Mozambique Channel

The surface circulation in the Mozambique Channel changes from the northern part, where the system is under dominance of the monsoons, to the central and southern parts, where mesoscale eddies are more frequent. Despite these differences in the physics between the three regions, satellite-based primary production shows that the whole region is characterized by similar seasonal variability of primary production, in which winter is the most productive season. A coupled physical-biogeochemical model configuration applied for the Mozambique Channel is used to investigate how wind stress and heat fluxes modulate seasonally and spatially the distribution of new and primary production in the region. Higher new production integrated over the euphotic layer depth in winter accounts for about 50% of the total primary production in the Mozambique Channel, indicating the seasonality of primary production is driven by new nutrients upwelled from bellow the euphotic zone. During the other seasons of the year the depth-integrated primary production is low, which is the period when the system depends on remineralization of the organic matter to sustain phytoplankton growth at the subsurface. Stronger wind stress is the dominant surface forcing in the northern part of the Mozambique Channel, which is responsible for vertical advection and entrainment of nitrate from below the euphotic zone that sustain the primary production during winter, while intense negative net heat flux is the dominant forcing in the central/southern parts. However, it is important to note that mesoscale eddies also enhance primary productivity in the region with a focus on the winter period.

Striking seasonal pattern of primary production in the river-dominated ocean margin of the northern South China Sea (NSCS-RiOMar) revealed by new field and remotely sensed data

Seasonal patterns of phytoplankton biomass and primary production (PP) are important features characterizing a marine ecosystem. The South China Sea (SCS) has been arbitrarily illustrated as an ecosystem with low PP in summer and high PP in winter. However, there is an emerging consensus about a lack of unity of PP seasonality in the SCS even in the northern SCS shelf-sea. Through analysis of the distributions of the remotely sensed colored dissolved organic matter absorption coefficient, a river-dominated ocean margin in the northern SCS (NSCS-RiOMar) with the 50 m isobath as its southeastern boundary has been defined. The difference in PP seasonality between the NSCS-RiOMar and the whole shelf-sea was evaluated by both the measured and the remotely sensed depth-integrated primary production (ΣPP). A diagnostic analysis found that euphotic depth and a function of the half-saturation light intensity accounted for the seasonal variation of ΣPP in the NSCS-RiOMar. Accordingly, the Platt and Sathyendranath (1993) model plus the nearest neighbor method and the semi-analytical estimates of euphotic depth were used to compute the remotely sensed ΣPP. The average ΣPP in the NSCS-RiOMar were 1146.4, 1604.7, 979.5, and 891.3 mg C m−2 d−1 in spring, summer, autumn, and winter, respectively, contrasted with small seasonal variations in the whole shelf-sea. The unimodal ΣPP peak was in June-July in accordance with the peak of Pearl River discharge. This result was striking because previous remote sensing studies all showed the reversed seasonal pattern of ΣPP in both the NSCS-RiOMar and the whole shelf-sea. The annual PP was estimated to be 33.5 ± 2.3 Mt C yr−1 and 56.2 ± 3.2 Mt C yr−1 in the NSCS-RiOMar and the whole shelf-sea, respectively. The nutrients inputted by the China Coastal Current might account for 4–23% of ΣPP in the NSCS-RiOMar between December and February.

The impact of temporal variability in light-climate on time-averaged primary production and a phytoplankton bloom in a well-mixed estuary

Phytoplankton primary production (PP) in turbid estuaries is often limited by light-availability. Two important factors altering light-climate are solar irradiance at the water surface and exponential light-extinction coefficient within the water column. Additionally, the depth of the water body changes the light-climate and corresponding PP by altering the ratio of the euphotic and mixing depth in a well-mixed estuary. These three parameters are highly variable yet are often assumed to be constant by both experimental scientists and modelers because of a lack of data or to reduce complexity. Because assuming constant parameters introduces an error, we utilize an idealized model of depth-integrated primary production to analyze the (individual) impact of temporal variability in these three parameters. We only consider the main tidal and solar constituents in temporal variability of the forcings and apply a second-order moment approximation to analyze the bias introduced to time-averaged PP estimates by neglecting temporal fluctuations. We demonstrate that the sign and magnitude of this bias are system-specific and depend on two non-dimensional parameters that characterize the system. The first is equivalent to the ratio of mixing and photic depth. The second accounts for typical incident irradiance and the photosynthetic parameters of the phytoplankton population present. To demonstrate the applicability of our approach, we apply the model to two cases in the Scheldt estuary (Belgium) in the brackish and freshwater part. In the first application, we study the impact of fluctuations on phytoplankton in dynamic equilibrium, where biomass is assumed to be constant. We show that variability in solar irradiance has the largest impact on time-averaged PP in dynamic equilibrium, resulting in a 30 percent decrease compared to time-invariant forcing. By comparing with a numerical integrator, we show that a second-order moment approximation correctly predicts the order of magnitude of the impact of temporal variability of the individual parameters. In the second application, we study the impact of fluctuations on unbounded exponential phytoplankton growth. Also here, fluctuations in solar irradiance have the largest impact and lead to a significant decrease in exponential growth. In this case study, we show that temporal fluctuations delay the onset of the biomass by two weeks and decrease the biomass by a factor 14 after two weeks compared to time invariant forcing. Additionally, we show that the temporal fluctuations induce low-frequency variability in phytoplankton biomass with similar periodicity as the spring-neap cycle, making it difficult to observe these phenomena in real-world time series.

Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections

Abstract. Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients, and changes to primary production, all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). A total of 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the multi-model global mean change (2080–2099 mean values relative to 1870–1899) ± the inter-model SD in sea surface temperature, surface pH, subsurface (100–600 m) oxygen concentration, euphotic (0–100 m) nitrate concentration, and depth-integrated primary production is +3.47±0.78 ∘C, -0.44±0.005, -13.27±5.28, -1.06±0.45 mmol m−3 and -2.99±9.11 %, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are +1.42±0.32 ∘C, -0.16±0.002, -6.36±2.92, -0.52±0.23 mmol m−3, and -0.56±4.12 %. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. The ESMs in CMIP6 generally project greater warming, acidification, deoxygenation, and nitrate reductions but lesser primary production declines than those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming increases upper-ocean stratification in CMIP6 projections, which contributes to greater reductions in upper-ocean nitrate and subsurface oxygen ventilation. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO2 concentrations than their RCP analogues for the same radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an increase in net primary production inter-model uncertainties in CMIP6, as compared to CMIP5.

Продукционные характеристики фитопланктона пролива Брансфилд в летний период

Production characteristics of summer phytoplankton in the Bransfield Strait were studied in January 2020 during the 79th cruise of the R/V «Akademik Mstislav Keldysh» in the Atlantic sector of the Southern Ocean. The primary production, concentration of chlorophyll «a» and the relative content of pheophytin «a» in the sum of chlorophyll «a» and pheophytin «a» were determined. The concentration of chlorophyll «a» within the upper 125 m layer varied from 0.04 to 1.29 mg/m3 with an average value of 0.61 mg/m3. The maximal values were detected in the upper 20 m layer in the northwestern part of the section. The integral content of chlorophyll in the photic layer ranged from 16.67 to 62.43 mg/m2 with an average content of 36.66 mg/m2, decreasing from west to east. The relative content of pheophytin «a» in the euphotic layer varied from 20 % to 68 % with an average value of 42 %. The lowest values of pheophytin, indicating active photosynthetic processes, were observed in the northwestern part of the section at depths of up to 30 m, in the surface horizon in the central part of the strait, and at depths up to 80 m at the southeastern stations. The depth-integrated primary production was equal to 116–206 mg C/m2 per day, the most active synthesis of organic matter was revealed in the surface horizon. The assimilation number was 0.33–0.45 μg C/μg Chl/hour*L, reaching maximum values in the upper ten-meter layer of water. The northwestern part of the Bransfield Strait was the most productive.

N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean

In the present study, we report N2 fixation rate (15N isotope tracer assay) and the diazotroph community structure (using the molecular method) in the western tropical North Pacific Ocean (WTNP) (13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N2 fixation activity and diazotroph community structure showed the dominance of unicellular N2 fixation over majority of the WTNP surface waters during the sampling periods. Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current (Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N2 fixation. In surface waters, volumetric N2 fixation rate (calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged (1.2±0.5) nmol/(L·d), with <10 μm size fraction contributed predominantly (88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N2 fixation rate over the upper 200 m ranged between 150 μmol/(m2·d) and 480 μmol/(m2·d) (average (225±105) μmol/(m2·d). N2 fixation can account for 6.2%±3.7% of the depth-integrated primary production, suggesting that N2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.

Multi-model remote sensing assessment of primary production in the subtropical gyres

Abstract The subtropical gyres occupy about 70% of the ocean surface. While primary production (PP) within these oligotrophic regions is relatively low, their extension makes their total contribution to ocean productivity significant. Monitoring marine pelagic primary production across broad spatial scales, particularly across the subtropical gyre regions, is challenging but essential to evaluate the oceanic carbon budget. PP in the ocean can be derived from remote sensing however in situ depth-integrated PP (IPPis) measurements required for validation are scarce from the subtropical gyres. In this study, we collected >120 IPPis measurements from both northern and southern subtropical gyres that we compared to commonly used primary productivity models (the Vertically Generalized Production Model, VGPM and six variants; the Eppley-Square-Root model, ESQRT; the Howard–Yoder–Ryan model, HYR; the model of MARRA, MARRA; and the Carbon-based Production Model, CbPM) to predict remote PP (PPr) in the subtropical regions and explored possibilities for improving PP prediction. Our results showed that satellite-derived PP (IPPsat) estimates obtained from the VGPM1, MARRA and ESQRT provided closer values to the IPPis (i.e., the difference between the mean of the IPPsat and IPPis was closer to 0; |Bias| ~ 0.09). Model performance varied due to differences in satellite predictions of in situ parameters such as chlorophyll a (chl-a) concentration or the optimal assimilation efficiency of the productivity profile (PBopt) in the subtropical region. In general, model performance was better for areas showing higher IPPis, highlighting the challenge of PP prediction in the most oligotrophic areas (i.e. PP

Comparison of air-sea CO2 flux and biological productivity in the South China Sea, East China Sea, and Yellow Sea: a three-dimensional physical-biogeochemical modeling study

Marginal seas play important roles in regulating the global carbon budget, but there are great uncertainties in estimating carbon sources and sinks in the continental margins. A Pacific basin-wide physical-biogeochemical model is used to estimate primary productivity and air-sea CO2 flux in the South China Sea (SCS), the East China Sea (ECS), and the Yellow Sea (YS). The model is forced with daily air-sea fluxes which are derived from the NCEP2 reanalysis from 1982 to 2005. During the period of time, the modeled monthly-mean air-sea CO2 fluxes in these three marginal seas altered from an atmospheric carbon sink in winter to a source in summer. On annualmean basis, the SCS acts as a source of carbon to the atmosphere (16 Tg/a, calculated by carbon, released to the atmosphere), and the ECS and the YS are sinks for atmospheric carbon (–6.73 Tg/a and–5.23 Tg/a, respectively, absorbed by the ocean). The model results suggest that the sea surface temperature (SST) controls the spatial and temporal variations of the oceanic pCO2 in the SCS and ECS, and biological removal of carbon plays a compensating role in modulating the variability of the oceanic pCO2 and determining its strength in each sea, especially in the ECS and the SCS. However, the biological activity is the dominating factor for controlling the oceanic pCO2 in the YS. The modeled depth-integrated primary production (IPP) over the euphotic zone shows seasonal variation features with annual-mean values of 293, 297, and 315 mg/(m2·d) in the SCS, the ECS, and the YS, respectively. The model-integrated annual-mean new production (uptake of nitrate) values, as in carbon units, are 103, 109, and 139 mg/(m2·d), which yield the f-ratios of 0.35, 0.37, and 0.45 for the SCS, the ECS, and the YS, respectively. Compared to the productivity in the ECS and the YS, the seasonal variation of biological productivity in the SCS is rather weak. The atmospheric pCO2 increases from 1982 to 2005, which is consistent with the anthropogenic CO2 input to the atmosphere. The oceanic pCO2 increases in responses to the atmospheric pCO2 that drives air-sea CO2 flux in the model. The modeled increase rate of oceanic pCO2 is 0.91 μatm/a in the YS, 1.04 μatm/a in the ECS, and 1.66 μatm/a in the SCS, respectively.

Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean

Abstract. The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilizing a unique experimental design that allowed for 24 h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters αB, PBmax and Ek. Mean values for each photosynthetic parameter under iron-replete conditions were 1.46 ± 0.55 (µg (µg Chl a)−1 h−1 (µM photons m−2 s−1)−1) for αB, 72.55 ± 27.97 (µg (µg Chl a)−1 h−1) for PBmax and 50.84 ± 11.89 (µM photons m−2 s−1) for Ek, whereas mean values under the control conditions were 1.25 ± 0.92 (µg (µg Chl a)−1 h−1 (µM photons m−2 s−1)−1) for αB, 62.44 ± 36.96 (µg (µg Chl a)−1 h−1) for PBmax and 55.81 ± 19.60 (µM photons m−2 s−1) for Ek. There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50° S, with very little difference observed in the subantarctic and polar frontal zone. These results emphasize the need for better parameterization of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

Relación entre la producción primaria y bacteriana pelágicas en dos lagunas húmicas vegetadas

In this study, we show that depth-integrated pelagic primary production (PP) can exceed bacterioplankton production (BP) in vegetated humic shallow lakes, giving as a result an autotrophic water column, despite light restrictions and availability of organic carbon for lake bacteria. Intuitively, these conditions should favor the development of a heterotrophic water column. Instead, during our survey, BP represented between 1.3 to 5% of PP most of the time. Only once, during late summer, BP was ~71% of PP. Although we cannot conclude about the mechanisms behind the observed results, previous surveys and experimentation in the wetland allow us to hypothesize that autotrophic conditions were favored by: i) the shallow nature of the lakes, which compensates for light attenuation by organic matter when integrating production in the water column, ii) the presence of anaerobic anoxygenic photosynthetic bacteria below the macrophyte cover, and iii) high predation rates on bacterioplankton by heterotrophic flagellates below the floating plants.

Nitrification and its influence on biogeochemical cycles from the equatorial Pacific to the Arctic Ocean.

We examined nitrification in the euphotic zone, its impact on the nitrogen cycles, and the controlling factors along a 7500 km transect from the equatorial Pacific Ocean to the Arctic Ocean. Ammonia oxidation occurred in the euphotic zone at most of the stations. The gene and transcript abundances for ammonia oxidation indicated that the shallow clade archaea were the major ammonia oxidizers throughout the study regions. Ammonia oxidation accounted for up to 87.4% (average 55.6%) of the rate of nitrate assimilation in the subtropical oligotrophic region. However, in the shallow Bering and Chukchi sea shelves (bottom ⩽67 m), the percentage was small (0–4.74%) because ammonia oxidation and the abundance of ammonia oxidizers were low, the light environment being one possible explanation for the low activity. With the exception of the shallow bottom stations, depth-integrated ammonia oxidation was positively correlated with depth-integrated primary production. Ammonia oxidation was low in the high-nutrient low-chlorophyll subarctic region and high in the Bering Sea Green Belt, and primary production in both was influenced by micronutrient supply. An ammonium kinetics experiment demonstrated that ammonia oxidation did not increase significantly with the addition of 31–1560 nm ammonium at most stations except in the Bering Sea Green Belt. Thus, the relationship between ammonia oxidation and primary production does not simply indicate that ammonia oxidation increased with ammonium supply through decomposition of organic matter produced by primary production but that ammonia oxidation might also be controlled by micronutrient availability as with primary production.

Vertical distribution of primary production and chlorophyll a in the Kara Sea

On the basis of data obtained during three ecosystem expeditions in the Kara Sea, the vertical variability of the primary production (PP) and chlorophyll a (Chl) in autumn was studied. The Chl maximum was detected mainly on the surface (Chl0). A homogenous Chl distribution in the euphotic layer (1% photo-synthetically available radiation) and a nearly linear decrease in the Chl concentration below this layer were observed in waters with Chl0 values of 0.1–0.5 mg/m3. In waters with Chl0 > 0.5 mg/m3, the Chl concentration in the studied layer decreased linearly or exponentially. The subsurface Chl maximum (SCM) was registered weekly and was detected mostly in waters with a Chl0 content of 0.1–0.5 mg/m3. The SCM formation in the Kara Sea was consistent with the general patterns for the World Ocean. Water-column stability, the content of biogenic elements, and the level of subsurface irradiance had an approximately equal effect on SCM formation. The contribution of the SCM to the depth-integrated PP varied from 1 to 27%. The parameterization of vertical profiles of Chl was performed in order to be used in depth-integrated PP models. The Chl maximum on the surface and the negligible SCM facilitate the estimation of depth-integrated PP on the basis of satellite data and the use of vertical-resolution models.

Evaluation of the influence of abiotic and biotic factors on primary production in the Kara Sea in autumn

A regression analysis of the parameters of primary production versus environmental factors was performed on the basis of data of three complex expeditions performed in the Kara Sea in September to October 1993, 2007, and 2011. The analysis of the dependence of the depth-integrated primary production (PPint) on the value of surface chlorophyll a (Chl0) and assimilation activity (ANm) showed that only 12% of the variability of the integrated PPint was determined by the variability of Chl0, whereas the correlation between PPint and ANm was strong (R2 = 0.635). Thus, in autumn, PPint values in the Kara Sea depended primarily on the activity of phytoplankton assimilation. At the end of the vegetative season, high (close to or above 1 mg/m3) Chl0 values did not reflect phytoplankton production within the entire photosynthetic layer, where organic matter was synthesized at a low rate. In turn, PPint and ANm depended primarily on the intensity of insolation and was weakly related to the content of dissolved forms of nitrogen and phosphorus. In autumn, at the end of the vegetative season, insolation apparently is the main factor in the determination of the formation of primary production in the Kara Sea.

Vertical spatio-temporal patterns of phytoplankton due to migration behaviors in two shallow, microtidal estuaries: Influence on phytoplankton function and structure

Vertical patterns of phytoplankton biomass driven by phytoflagellate migration constitute a natural, short-term process with potentially important impacts on estuarine primary production and community composition. Although often considered well-mixed, phytoflagellate vertical migration patterns have been observed in shallow estuaries. We investigated vertical migration patterns in two shallow estuaries to determine how often and under what environmental conditions vertical migration patterns occurred, and to understand the potential impacts of vertical migration on phytoplankton composition and productivity. Vertical migration patterns were determined from circumannual records of decimeter-scale, semihourly, vertical profiles of chlorophyll fluorescence in two shallow, microtidal estuaries, the New River Estuary and Neuse River Estuary, North Carolina, USA. Observed migration patterns were compared with coincident measures of temperature, salinity, light, turbidity, nutrients, vertical stratification, and wind speed. A simple light × biomass model was used to estimate the influence of vertical migration patterns on total water column primary production. Collectively, between two sites in each estuary, diel vertical migration (DVM) patterns were detected on about half of the days. A secondary migration pattern reflecting a midday descent was also observed and was attributed to avoidance of intense midday surface irradiance. The likelihood of detectable DVM patterns increased under warmer conditions and lower incident irradiance, and at two of the stations was lessened by elevated wind speeds or reduced stratification intensity. Even weak stratification may be sufficient to reduce vertical mixing to levels that allow effective depth regulation by directed swimming. Modeled depth-integrated primary production was increased modestly (<10%) by observed migration patterns compared to a hypothetical vertically-homogenous biomass distribution. Access to optimal light levels and elevated bottom water dissolved inorganic nitrogen concentrations provides selective advantages that may explain phytoflagellate dominance in these estuaries, particularly during the warmer months, when production in surface waters of both systems is strongly N-limited.

Patterns of the Kara Sea primary production in autumn: Biotic and abiotic forcing of subsurface layer

Primary production and fundamental environmental factors were measured during September–October 1993, 2007 and 2011 in the Kara Sea. Relationships between the depth-integrated primary production (PPint), the surface chlorophyll a (Chl0) concentration and the maximum chlorophyll specific carbon rate within water column (Pbopt) had shown that only 12% of PPint variability were determined by Chl0 and there were strong correlations between PPint and Pbopt (R2=0.64). Thus, in the autumn PPint values were largely influenced by the phytoplankton assimilation activity. At the end of a vegetative season high (close or above 1mgm−3) values of Chl0 were not an index of phytoplankton productivity within photosynthetic layer where the organic matter synthesis rate was low which testifies the lack of correlation between Chl0 and PPint (R2=0.12) and between Chl0 and depth-integrated chlorophyll a (chl a) (R2=0.22). This conclusion is based on the low assimilation activity within water column and small thickness of the photosynthetic layer. The latter corresponds to low insolation and water transparency. The lack of correlation between surface and depth-integrated productivity parameters is the challenge for satellite PPint estimations. In turn PPint and Pbopt depended mainly on photosynthetically available radiation (PAR) and weakly related to the nitrogen and phosphorus concentrations. At the end of a vegetative season PAR level, apparently, should be considered as the main factor for primary production in the Kara Sea. Comparison between the integrated primary production from shipboard in September and the simulated evaluations of productivity (PP models) suggests that PP models overestimate the in situ PPint by a factor of 3–7 in the different Kara Sea regions. Improving of Kara Sea primary production estimations implies the development of regional satellite chl a algorithm and local primary production model considering specific features of PP in this Arctic Ocean region.