Subsurface Biomass Maximum Research Articles

Biogeochemical Argo Floats Reveal the Evolution of Subsurface Chlorophyll and Particulate Organic Carbon in Southeast Indian Ocean Eddies

AbstractEddies modulate open ocean productivity, and this influence depends on both eddy source and evolution. Southeast Indian Ocean eddies are important pathways for the westward transport of biogeochemical anomalies from the Leeuwin current into the central oligotrophic South Indian Ocean (SIO). Eddy processes at the base of the mixed layer may stimulate and sustain phytoplankton, allowing these eddy impacts to persist over thousands of kilometers. We present 4 months of high‐frequency profiles from autonomous floats in one anticyclonic and one cyclonic eddy in the SIO. At the start of observations, from September to October, particulate organic carbon (POC) and especially chlorophyll were higher in the cyclone, and evenly distributed throughout the mixed layers in both eddies. As spring progressed and the eddies were transported westward, chlorophyll and POC concentrated at the base of the mixed layer at depths invisible to satellites, likely reflecting nutrient depletion in overlying waters. In the anticyclone, the increased chlorophyll at depth occurred as POC decreased, suggesting photo‐acclimation and thus both light and nutrient stress. In contrast, in the cyclone chlorophyll to POC ratios remained close to constant as their subsurface maxima formed. In both eddies, the subsurface biomass maxima exhibited no significant change in oxygen saturation state over several months suggesting these communities are sustained by low ongoing productivity in balance with community respiration. Thus, deep biomass layers may represent a mechanism for long‐distance transfer of eddy plankton communities which is not reflected in satellite remote sensing.

Changes in water column oxygen, estimates of productivity and the development of anoxia in a major embayment of the southern Benguela eastern boundary upwelling system

The metabolic balance of autotrophy and heterotrophy in St Helena Bay was investigated through estimates of net community production (NCP) and respiration (R), as determined by changes in water column oxygen. Compared to adjacent coastal waters St Helena Bay was shown to be an area of low-enrichment and high-retention. Productivity during the upwelling season was found to be nutrient limited in the upper few metres (0–3 m) and light limited through self-shading below these surface waters. During the upwelling season the maintenance of thermal stratification in the bay leads to consistently low nutrients (nitrate x¯ = 1.41 mmol m−3) in the surface waters (0–3 m). High biomass maintained through mechanisms of cyclonic retention and entrainment from the Namaqua shelf underlies the rapid attenuation of light through the water column and the consequent limitation of subsurface productivity. As a result productivity maxima were located in surface waters (0 or 3 m) irrespective of the low nutrient concentrations in these waters and the presence of subsurface biomass maxima. Estimates of gross community production (GCP) normalised to Chl a therefore declined sharply below 3 m depth. The mean community compensation depth (Zcc), where R = GCP, was determined to be 9.16 m and was shallower than the 1% light depth for PAR (Z1%PAR) at 15.42 m. The role of light in limiting productivity was clearly evident in that 48% of water column biomass was located below the Zcc. The importance of the light environment in controlling the seasonality of productivity was further evident in that daily rates of productivity in surface waters normalised to Chl a were clearly linked to day length. Phytoplankton community composition and successional status were also shown to influence productivity with the contribution of R to GCP increasing with the succession of phytoplankton communities. Estimates of mean water column Chl a (8.60 mg m−3 as determined from discrete water samples and 8.40 mg m−3 as estimated from profiles of fluorescence) and mean integrated water column productivity (5.38 g C m−2 d−1 as determined by the light-and-dark bottle oxygen method) were ~ 1.5 fold higher than past measurements most of which were made during the late 1970s and 80s. Oxygen depletion within St Helena Bay is locally enhanced as a result of the entrainment and retention of biomass in St Helena Bay and causes it to function as an area of higher carbon sequestration.

Biological production in two contrasted regions of the Mediterranean Sea during the oligotrophic period: an estimate based on the diel cycle of optical properties measured by BioGeoChemical-Argo profiling floats

Abstract. This study assesses marine community production based on the diel variability of bio-optical properties monitored by two BioGeoChemical-Argo (BGC-Argo) floats. Experiments were conducted in two distinct Mediterranean systems, the northwestern Ligurian Sea and the central Ionian Sea, during summer months. We derived particulate organic carbon (POC) stock and gross community production integrated within the surface, euphotic and subsurface chlorophyll maximum (SCM) layers, using an existing approach applied to diel cycle measurements of the particulate beam attenuation (cp) and backscattering (bbp) coefficients. The diel cycle of cp provided a robust proxy for quantifying biological production in both systems; that of bbp was comparatively less robust. Derived primary production estimates vary by a factor of 2 depending upon the choice of the bio-optical relationship that converts the measured optical coefficient to POC, which is thus a critical step to constrain. Our results indicate a substantial contribution to the water column production of the SCM layer (16 %–42 %), which varies largely with the considered system. In the Ligurian Sea, the SCM is a seasonal feature that behaves as a subsurface biomass maximum (SBM) with the ability to respond to episodic abiotic forcing by increasing production. In contrast, in the Ionian Sea, the SCM is permanent, primarily induced by phytoplankton photoacclimation, and contributes moderately to water column production. These results clearly demonstrate the strong potential for transmissometers deployed on BGC-Argo profiling floats to quantify non-intrusively in situ biological production of organic carbon in the water column of stratified oligotrophic systems with recurring or permanent SCMs, which are widespread features in the global ocean.

DMS emissions from the Arctic marginal ice zone

Phytoplankton blooms in the Arctic marginal ice zone (MIZ) can be prolific dimethylsulfide (DMS) producers, thereby influencing regional aerosol formation and cloud radiative forcing. Here we describe the distribution of DMS and its precursor dimethylsulfoniopropionate (DMSP) across the Baffin Bay receding ice edge in early summer 2016. Overall, DMS and total DMSP (DMSPt) increased towards warmer waters of Atlantic origin concurrently with more advanced ice-melt and bloom stages. Relatively high DMS and DMSPt (medians of 6.3 and 70 nM, respectively) were observed in the surface layer (0–9 m depth), and very high values (reaching 74 and 524 nM, respectively) at the subsurface biomass maximum (15–30 m depth). Microscopic and pigment analyses indicated that subsurface DMS and DMSPt peaks were associated with Phaeocystis pouchetii, which bloomed in Atlantic-influenced waters and reached unprecedented biomass levels in Baffin Bay. In surface waters, DMS concentrations and DMS:DMSPt ratios were higher in the MIZ (medians of 12 nM and 0.15, respectively) than in fully ice-covered or ice-free conditions, potentially associated with enhanced phytoplanktonic DMSP release and bacterial DMSP cleavage (high dddP:dmdA gene ratios). Mean sea–air DMS fluxes (µmol m–2 d–1) increased from 0.3 in ice-covered waters to 10 in open waters (maximum of 26) owing to concurrent trends in near-surface DMS concentrations and physical drivers of gas exchange. Using remotely sensed sea-ice coverage and a compilation of sea–air DMS flux data, we estimated that the pan-Arctic DMS emission from the MIZ (EDMS, MIZ) was 5–13 Gg S yr–1. North of 80°N, EDMS, MIZ might have increased by around 10 ± 4% yr–1 between 2003 and 2014, likely exceeding open-water emissions in June and July. We conclude that EDMS, MIZ must be taken into account to evaluate plankton-climate feedbacks in the Arctic.

Saldanha Bay, South Africa II: estimating bay productivity

Saldanha Bay is a narrow-mouth bay on the west coast of South Africa linked to the southern Benguela upwelling system. Bay productivity was investigated by use of the conventional light-and-dark bottle oxygen method, and, for comparison, through assimilation of the stable isotope tracer 13C. Gross community production GCP and net community production NCP, as determined from the oxygen method, were respectively 2.6 and 2.4 times higher than estimates determined from the stable isotope method. Chlorophyll a (Chl a) concentrations increased with the onset of spring and well-defined subsurface maxima developed in association with increasingly stratified conditions (mean water column Chl a concentrations ranged from 5.4 to 31.5 mg m−3 [mean 15.5 mg m−3; SD 7.6]). A sharp decline in photosynthetic rates P* (GCP normalised to Chl a concentration) with depth was attributed to light limitation, as demonstrated by the high vertical attenuation coefficients for downward irradiance Kd, which varied from 0.29 to 0.70 m−1 (mean 0.48 m−1; SD 0.12). Productivity maxima were consequently near-surface despite the presence of deeper subsurface biomass maxima. The community compensation depth Zcc, where gross community production balances respiratory carbon loss for the entire community, ranged from 2.9 to 9.2 m (mean 5.8 m; SD 2.2), and was typically shallower than the 1% light depth for PAR (photosynthetically available radiation), Z1%PAR, which is traditionally assumed to be the depth of the euphotic zone and which ranged from 6.6 to 15.9 m (mean 9 m; SD 2.6). Autotrophic communities, where organic matter is produced in excess of respiratory demand, were confined on average to the upper 5.8 m of the water column, and often excluded the bulk of the phytoplankton community, where light limitation is considered to lead to heterotrophic community metabolism. Estimates of integrated water column productivity ranged from 0.84 to 8.46 g C m–2 d−1 (mean 3.35 g C m−2 d−1; SD 1.9).

Bimodal distribution patterns of motile phytoplankton in relation to physical processes and stratification (Gulf of Finland, Baltic Sea)

The dynamics and vertical distribution patterns of chlorophyll a and motile phytoplankton species in relation to the vertical stratification and its spatial and temporal variations were analyzed on the basis of observational data collected in the Gulf of Finland in July 2010. Bimodal vertical distribution of phytoplankton characterized by a thick maximum in the upper 10m layer and a thin maximum in the deeper part of the thermocline, where the chlorophyll a fluorescence values often exceeded those in the upper layer, was observed in the areas of locally weaker stratification at the mesoscale in the second half of July. We suggest that the observed bimodal distribution pattern was a result of the downward migration of phytoplankton through the thermocline at night and asynchronous upward movement of cells with a migration cycle longer than 24h. The main species found in the sub-surface maxima were the dinoflagellates Heterocapsa triquetra and occasionally Dinophysis acuminata. Biomass of H. triquetra increased in the surface layer concurrently with the appearance of sub-surface biomass maxima under conditions of relatively high horizontal variability of vertical stratification at the mesoscale. It supports our suggestion that the mesoscale dynamics favors successful vertical migration of this species between the surface layer and deep nitrate reserves. Sub-surface maxima of phytoplankton biomass, as well as vertical migration, leading to selective transport of nutrients, have to be taken into account in the regional ecosystem models, both to forecast phytoplankton blooms and describe more precisely the seasonal dynamics of nutrients and phytoplankton primary production in the stratified estuaries.

Vertical dynamics of summer phytoplankton in a stratified estuary (Gulf of Finland, Baltic Sea)

We present the results of multiparametric observations designed to follow the phytoplankton dynamics and interrelated physical, chemical and biological processes in the Gulf of Finland (Baltic Sea). Data were acquired by an autonomous moored water column profiler, an acoustic Doppler current profiler, a flow-through system installed aboard a ferry and by profiling and discrete water sampling aboard research vessels in July and August 2009. The main aim of the study was to investigate the processes responsible for the formation and maintenance of sub-surface maxima of phytoplankton biomass. We suggest that the environmental conditions caused by the prevailing atmospheric and oceanographic forcing (wind; vertical stratification; basin-wide, mesoscale and sub-mesoscale processes) are preferred by certain species/taxonomic groups and explain the migration patterns of phytoplankton. Nocturnal downward migration of phytoplankton with a swimming speed up to 1.6 m h−1 occurred when the community was dominated by the dinoflagellate Heterocapsa triquetra. The observed splitting of the population into two vertically separated biomass maxima suggests that the H. triquetra cells, which reached the sub-surface layers with high nutrient concentrations, experienced bi-diurnal or asynchronous (when swimming upwards) vertical migration. The most intense sub-surface biomass maxima, on some occasions with the biomass much higher than that in the surface layer, were detected in connection to the sub-mesoscale intrusions below the depth of the strongest vertical density gradient.

Prevalence, structure and properties of subsurface chlorophyll maxima in Canadian Arctic waters

Comprehensive investigations of the Canadian Arctic during late summer and early fall revealed the widespread occurrence of long-lived subsurface chlorophyll maxima (SCM) in season- ally ice-free waters. The vertical position of the SCM corresponded with the depth of the subsurface biomass maximum (SBM), at least in Baffin Bay, suggesting that SCM could be an important source of carbon for the food web. Most of these SCM were located well below the pycnocline in close association with the nitracline, implying that their vertical position was driven mainly by a shortage of inorganic nitrogen in the upper euphotic zone. The diversity of SCM configurations with respect to physical properties of the water column complicates the estimation of euphotic-zone chlorophyll and primary production from surface properties. High photosynthetic yields (Fv/Fm) showed the phyto- plankton to be photosynthetically competent and well acclimated to conditions of irradiance and nutrient supply near the surface and at the SCM. A well-defined primary nitrite maximum was associated with the SCM in the southwest Canadian Arctic, but not in the northeast where nitrite concentrations were highest much below the euphotic zone. This contrast is consistent with differences in vertical stratification, the light-dark cycle and, possibly, the physiological state and taxonomic composition of the phytoplankton community at the SCM. This study demonstrates that the SCM, once regarded as anecdotal due to under-sampling, are a dominant feature of the Arctic Ocean that should be considered in remote sensing studies and biogeochemical models.

Revisiting subsurface chlorophyll and phytoplankton distributions

Vertical profiles of chlorophyll concentration and phytoplankton biomass at ALOHA (HOT) are analyzed for the time period 1988 to 2004. Two different methods are applied: in the standard approach the data are averaged over depth horizons and in the alternative approach the profiles are shifted to the depth of the deepest subsurface maximum before averaging. The results show that the latter is the only meaningful way to look at vertical distribution patterns of both chlorophyll and phytoplankton in the oligotrophic ocean. In particular, a pronounced subsurface maximum of phytoplankton biomass appears only if this depth-adjustment method is used. Otherwise the vertical displacement of the subsurface biomass due to changes in the subsurface light field masks the actual signal: the thickness of the subsurface maximum is overestimated and the maximum is reduced. The results of this study have far-reaching consequences for the interpretation of the large number of profiles of chlorophyll and phytoplankton in the oligotrophic ocean. The absence of a subsurface biomass maximum might not be necessarily a result of photoacclimation but of inadequate analyses combined with coarse vertical resolution.

Phytoplankton distribution and survival in the thermocline

Observations of the vertical structure of density, concentrations of chlorophyll a and nitrate, and turbulent dissipation rates were made over a period of 25 h in a well‐stratified shelf region in the Western English Channel, between neap and spring tides. Maximum turbulent dissipation at the base of the thermocline occurred almost 5 h after maximum tidal currents. This turbulence aids phytoplankton growth by supplying bottom‐layer nutrients into the subsurface chlorophyll maximum but reduces phytoplankton concentrations in the thermocline by mixing cells from the base of the subsurface maximum into the bottom mixed layer. The turbulent dissipation observations were used to estimate an average nitrate flux into the thermocline of 2.0 (0.8‐3.2, 95% confidence interval) mmol m−2 d−1, which is estimated to have been capable of supporting new phytoplankton growth at a rate of 160 (64–256) mg C m−2 d−1. Turbulent entrainment of carbon from the base of the subsurface biomass maximum into the bottom mixed layer was observed to be 290 (120–480) mg C m−2 d−1. This apparent excess export from the chlorophyll maximum is suggested to be a feature of the spring‐neap cycle, with export dominating as the tidal turbulence increases toward spring tides and erodes the base of the thermocline. The observed rate of carbon export into the bottom mixed layer could account for as much as 25% of the gross annual primary production in stratifying shelf seas. Such turbulent losses, combined with grazing losses and low light levels, suggest that phytoplankton need to be highly adapted to environmental conditions within the thermocline in order to survive.

Investigating the seasonal vertical structure of phytoplankton in shelf seas

A 1-dimensional (vertical) physical-biological coupled model is presented. The model is designed for investigations into the link between the vertical turbulent structure of a coastal or shelf sea water column, and the primary production. The physical model employs a turbulence closure scheme to provide the link between local vertical stability (driven by seasonal solar heating) and the vertical turbulent mixing (driven by tidal currents and surface wind stress). The biological component of the model is a simple cell quota, threshold limitation model, with either 1 or 2 taxa/species of phytoplankton growing in response to light and dissolved inorganic nitrogen. The user has control over all model physical and biological driving parameters. Graphical screen output, suitable for basic visualisation and teaching purposes, is generated as the model operates, and more detailed data are written to files for later analysis. In order to demonstrate the model's use in simple hypothesis investigation, an example of the model operation is illustrated. This focuses on the effect that the springs-neaps tidal cycle has on the production within the summer sub-surface biomass maximum, illustrating the fortnightly input of new nitrogen into the thermocline and the subsequent new production.

Nutrients, irradiance, and mixing as factors regulating primary production in coastal waters impacted by the Mississippi River plume

Relationships among primary production, chlorophyll, nutrients, irradiance and mixing processes were examined along the salinity gradient in the Mississippi River outflow region. A series of six cruises were conducted during 1988–1992 at various times of year and stages of river discharge. Maximum values of biomass and primary production were typically observed at intermediate salinities and coincided with non-conservative decreases in nutrients along the salinity gradient. Highest values of productivity (>10 gC m −2 d −1) and biomass (>30 mg chlorophyll a m −3) were observed in April 1988, July–August 1990 and April–May 1992; values were lower in March and September 1991. Rates of primary production were apparently constrained by low irradiance and mixing in the more turbid, low salinity regions of the plume, and by nutrient limitation outside the plume. Highest values of primary production occurred at stations where surface nutrient concentrations exhibited large deviations from conservative mixing relationships, indicating that depletion of nutrients was related to phytoplankton uptake. Mixing and advection were important in determining the location and magnitude of primary production maxima and nutrient depletion. In addition to growth within plume surface waters, enhanced growth and/or retention of biomass may have occurred in longer residence time waters at the plume edge and/or beneath the surface plume. Vertical structure of some plume stations revealed the presence of subsurface biomass maxima in intermediate salinity water that was depleted in nutrients presumably by uptake processes. Exchange between subsurface water and the surface plume apparently contributed to the reduction in nutrients at intermediate salinities in the surface layer. DIN (=nitrate+nitrite+ammonium) : PO 4 (=phosphate) ratios in river water varied seasonally, with high values in winter and spring and low values in late summer and fall. Periods of high DIN : PO 4 ratios in river nutrients coincided with cruises when surface nutrient concentrations and their ratios indicated a high probability for P limitation. N limitation was more likely to occur at high salinities and during late summer and fall. Evidence for Si limitation was also found, particularly in spring.

Vertical patterns of phytoplankton size distribution in the Cantabric and Balearic Seas

Abstract In this paper we analyze plankton size distribution (2–100 μm of equivalent spherical diameter) under different hydrographic structures, such as the seasonal thermocline of the Cantabric and Balearic seas and the oceanographic front present in the Balearic Sea. Nannoplanktonic fraction (2–20 μm) tends to present biomass maxima at shallower depths than the chlorophyll maximum in stratified waters, as well as in the more productive waters near a front. The slope of the normalized size-biomass spectrum was more negative in these zones of phytoplankton biomass maxima indicating a higher proportion of smaller cells. The main hypotheses to explain chlorophyll and biomass maxima as well as phytoplankton accumulation in productive areas are reviewed. According to the observed results, the most suitable mechanism to explain subsurface biomass maxima is the active net growth of the smaller phytoplanktonic cells. The importance of the methodology employed for analyzing the size distribution of plankton communities and its relationship with hydrographic variability are also discussed.

Relative Importance of Habitat Variables in Predicting the Distribution of Phytoplankton at the Ecotone of the Antarctic Upwelling Ecosystem

The vertical distribution of phytoplankton standing crop, number of species, and community diversity in five transects across the ecotone of the Antarctic Convergence were studied in relation to physical and chemical habitat variables (light, temperature, salinity, density, relative stability, oxygen, phosphate, nitrate, and silicate) and to bacteria and yeasts. Two distinct phytoplankton communities were observed, one north and one south of the convergence. Within the convergence the southern diatomaceous Antarctic community sinks with the formation of Antarctic Intermediate Water and can be traced underneath the northern dinoflagellate subantarctic community by use of indicator species. Chaetoceros bulbosum, Chaetoceros dichaeta, Dactyliosolen antarctica, and Eucampia zoodiacus were used as indicators of the Antarctic community, and Rhizosolenia delicatula, Amphidinium amphidinoides, Oxytoxum variabile, and Phalacroma pulchellum represented the subantarctic community. Submergence of the Antarctic community is corroborated by the vertical distribution of phytoplankton biomass. South of the convergence in the upwelling system, biomass maxima of the phytoplankton consist of Antarctic species. Within the convergence separate subsurface biomass maxima contain the Antarctic community, and both Antarctic and subantarctic species constitute surface maxima of standing crop. In the upper water of the oceanic system north of the convergence, subantarctic species make up most of the biomass, where as small numbers of Antarctic species were observed in the aphotic zone. Surface layers of the euphotic zone in both systems were dominated by a small number of species. Maxima of species number occurred deeper than those of biomass, possibly reflecting the result of competitive exclusion. A low community diversity was observed in the upper water of each area, and a high ecotonal diversity was found at the convergence. A linear multiple regression analysis showed the relative importance of habitat variables in predicting the vertical distribution of the phytoplankton both within the oceanic and upwelling systems and within their transition zone. Water—mass structure, turbulence, light, silicate, and an index of heterotrophic conditions can be used to predict up to 66% of phytoplankton variation across the convergence. Additional prediction can only be achieved by incorporation of biological rate data and nonlinear expression of variable interaction in simulation models based on the present analysis.