Physical drivers of phytoplankton production in the southern Benguela upwelling system

Investigations of primary production (PP) were undertaken in the southern Benguela ecosystem during two research surveys in October 2006 and May 2007. Significant differences in environmental conditions, as well as biomass and PP, were observed between October and May. During October, integrated biomass and PP were significantly higher, ranging from 20.43 to 355.01 mg m⁻², and 0.71 to 6.98 g C m⁻² d⁻¹, respectively, than in May, where the range was 47.92–141.79 mg m⁻², and 0.70–3.35 g C m⁻² d⁻¹, respectively. Distribution patterns indicated low biomass and PP in newly upwelled water along the coast, higher biomass and PP in the mid-shelf region, while lower values were observed at and beyond the shelf edge. Latitudinal variations showed consistently higher biomass and PP in the St. Helena Bay region compared to biomass and PP south of Cape Town. During both surveys, phytoplankton communities were comprised primarily of diatoms and small flagellates, with no significant differences. Phytoplankton adaptation to environmental variability was characterised by increased P_m^B and E_k under elevated temperatures and irradiance, while no clear relationships were evident for α^B. Generalised Additive Models (GAMs) showed that photosynthetic parameters were all significant predictors of photosynthesis rates (P_z), with P_m^B being the most important, accounting for 36.97% of the deviance in P_z. However, biomass levels and environmental conditions exerted a much greater influence on P_z, with irradiance explaining the largest proportion (68.24%) of the deviance. Multiple predictor GAMs revealed that 96.26% of the deviance in P_z could be explained by a model which included nitrate, chlorophyll a, and irradiance.     

Photosynthesis versus irradiance relationships in the Atlantic sector of the Southern Ocean

Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO^–₃), regenerated production (ρNH⁺₄), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance (P vs E) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity (P^B_max) and low quantum efficiency of photosynthesis (α^B) for ρNO₃, but high P^B_max and α^B for ρNH₄, with consequently low export efficiency. Results at stations likely relieved of Fe stress (associated with shallow bathymetry and the marginal ice zone) showed the highest rates of P^B_max and α^B for ρNO₃ and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on P vs E parameters, using surface temperature, chlorophyll-a concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO₃) and α^B for ρNO₃ suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NO₃ reservoir. However, the observed relationships between community structure and the P vs E parameters suggest superior performance by smaller-sized cells, in terms of resource acquisition and Fe limitation, as the probable driver of smaller-celled phytoplankton communities that have reduced photosynthetic efficiency and which require higher light intensities to saturate uptake. A noticeable absence in covariances between chlorophyll-a and α^B, between P^B_max and α^B, and between temperature and α^B may have important implications for primary-production models, although the absence of some expected relationships may be a consequence of the small dataset and low range of variability. However, significant relationships were observed between ambient NO₃ and α^B for ρNO₃, and between the light-saturation parameter E_k for ρNO₃ and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO₃ reservoir and that larger-celled communities are more efficient at fixing NO₃ in low light conditions. Although the mean P^B_max results for ρC were consistent with estimates of global production from satellite chlorophyll measurements, the range of variability was large. These results highlight the need for more-advanced primary-production models that take into account a diverse range of environmental and seasonal drivers of photosynthetic responses.     

Picoplankton: Contribution to Phytoplankton Production in the Strait of Messina*

Abstract. Biomass and primary production rates derived from photosynthesis-light curves on picoplankton (< 1 um fraction) and total phytoplankton were compared for the Strait of Messina. Picoplankton biomass ranged between 0.063 and 0.094 mg Chi a m^-5 and accounted for 56–63 % of the total. Total primary production rates were between 0.22 and 1.56 mg C-m^-2-h^-1 of which the picoplankton contribution ranged from 24 to 43%. In this turbulent nearshore environment, the contribution of picoplankton to total phytoplankton production is considerably less than in calmer open-ocean waters. Carbon assimilation numbers (Pm^b), the initial slope (a), adaptation parameter (I_k), optimal irradiance (I_m) and compensation intensity (L_m,) for the picoplankton were lower than for the > 1 μm fraction. Fitted respiration (Rm^B) for the picoplankton was, however, on the average higher (10.3 % of Pm^B) than for the > 1 μrn fraction (6.6 %). Assimilation numbers for the > 1 μm fraction in the southern stations were of higher magnitude (8.5–12.0 mg C mg Chi a^-1 h^-1) than in the northern station, possibly due to the impact of upwelled water flowing southward along the Sicilian coast.     

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 K_d, 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 Z_cc, 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).     

Acclimations of macroalgae as reflected in photosynthetic parameters derived from PAM fluorometry, and possible implications for abundance patterns

The presences of the common macroalgae Ulva sp. and Jania rubens vary between seasons along the Israeli Mediterranean intertidal zone. To reveal some of the potential acclimation mechanisms of these algae, we examined their photosynthetic traits during the year using rapid light curves (RLC) derived from pulse amplitude modulated (PAM) fluorometry. In addition, the relationships between those photosynthetic traits and the relative abundances of the two algae were investigated. Ulva sp. showed high maximal electron transport rates (ETR_max) (49.7–68.8 μmol electrons m⁻² s⁻¹) and onsets of light saturation values (E_k) (75.8–85.6 μmol photons m⁻² s⁻¹) in the winter months of December–February, while low values were found in the summer months of June–July (5.2–20.6 μmol electrons m⁻² s⁻¹ and 6.0–23.7 μmol photons m⁻² s⁻¹, respectively). At noon time (during the highest irradiance of the day), the maximal effective quantum yield (Y₀) did not vary significantly during the year. These results indicate that seasonal irradiance influences the number of reaction centres per thallus area. Both algae showed depressions in Y₀ at noon (23.8% for Ulva sp. and 20.3% for Jania rubens), indicating an efficient non‐photochemical quenching mechanism. A positive correlation between the relative abundance and the photosynthetic parameter Y₀ was found for Jania rubens when Y₀ was sampled in the morning or noon, indicating that growth rate and primary production for this alga can be estimated from RLCs. No such correlation was found for Ulva sp., indicating that, in addition to photosynthetic traits, the relative seasonal abundance of this alga is influenced by other factors such as grazing and/or catastrophic events.     

Nitrogen utilisation by size‐fractionated phytoplankton assemblages associated with an upwelling event off Westland,New Zealand

Abstract

Uptake rates of ammonium (NH₄ ⁺ ), nitrate (NO₃ ^? ), and urea by three subgroups of phytoplankton (< 200, < 20, < 2 μm) off Westland, were measured using ¹⁵ N tracer techniques in midwinter 1988, after a recent upwelling. For all size fractions at surface irradiance (I₁₀₀), nitrogen (N) was taken up primarily as NO₃ ^?. This accounted for 67–85% of total N uptake (SρN), whereas at 40 and 7% of surface irradiance, the regenerated N (NH₄ ⁺ ) and urea) made up 31–72% of SρN. Depth profile experiments for all three size components showed that uptake of NO₃ ^? was most light‐sensitive, followed by that of NH₄ ⁺ and urea. The irradiance and nutrient availability plot indicated that light was substantially more important than the nutrient concentrations in controlling the assimilation of N by microplankton (20–200 μm). Nano‐ (2–20 μm) and picoplankton (< 2 μm) however, were not as sensitive to either light or nutrient concentrations. High winds and the resulting deep mixing, combined with offshore and alongshore advection in the midwinter, were suggested to be the major cause of the low biomass and N productivity.     

On the interest of using field primary production data to calibrate phytoplankton rate processes in ecosystem models

In many ecosystem models based on empirical formulations, parameters generally are calibrated in order to achieve the best fit between measured and simulated chlorophyll a standing stocks. An accurate calibration of rate processes as primary production rarely is taken into account. In this paper, we test the usefulness of calibration of phytoplankton photosynthetic processes in an ecosystem model using field primary production data. We used 18 months of photosynthetic process data from the Baie des Veys ecosystem (Normandy, France). Five empirical formulations of photosynthesis–irradiance curve models amongst the most widely used were tested. In each formulation, the variability of photosynthetic parameters (i.e. the light-saturated rate of photosynthesis (P_max^B) and the initial slope of the photosynthesis–light curve (α^B)) was considered depending on environmental factors (temperature and nutrient availability). The fit of the five equations as well as the calibration of parameters on field measurements (i.e. the light-saturated rate of photosynthesis (P_ref^B), the initial slope of the photosynthesis–light curve (α_ref^B), the half-saturation constant for nitrogen (K_N) and silicates uptake (K_Si), and the coefficient in the exponential thermal effect (K_T)) was performed using the whole available data set of P vs. E curves (n = 143, P vs. E curves). Then, the Smith formulation allowing the best simulation of the Baie des Veys primary production and corresponding parameters were introduced in an ecosystem box model. This formulation led directly to a satisfactory representation of the Baie des Veys phytoplankton dynamics without additional calibration. Results obtained were compared with a more classical approach in which ecosystem models were calibrated using published values of parameters. This comparison showed that for the two years studied, annual primary production estimated through the ecosystem model was 13% and 26% higher with our approach than with the more classical approach. This work emphasizes the importance of accurately representing rate processes in ecosystem models in order to adequately simulate production as well as standing stocks.     

Growth, production, and mortality of the chemosynthetic vesicomyid bivalve, Calyptogena kilmeri from cold seeps off central California

The vesicomyid Calyptogena kilmeri is one of the most abundant bivalves inhabiting chemosynthetic environments shallower than c. 1500 m along central California. We estimated the population size structure, biomass, rates of individual growth, somatic production, and mortality for C. kilmeri, based on sampling of seep habitats and tag–recapture studies at chemosynthetic communities in Monterey Bay, California. The composite growth rate of C. kilmeri over all sites was relatively high (K = 0.25), reaching c. 80% of asymptotic length (104.7 mm) in 6.6 years. The density of C. kilmeri was estimated as 938.5 ind. m⁻² and biomass density varied from 704 to 2059 g ash free dry mass (AFDM) m⁻². Somatic production was also high (294–297 g·AFDM·m⁻² year⁻¹), and production/biomass ratios for C. kilmeri varied from 0.14 to 0.42 among sites, related to variation in size–frequency distributions among sites. Instantaneous mortality rates estimated from size distributions ranged from 0.17 to 0.24 year⁻¹. Growth and somatic production by C. kilmeri are in the range reported for chemosynthetic bivalves from hydrothermal vent and seep habitats, as well as photosynthetic‐based assemblages of inshore or intertidal bivalves, and greatly exceed rates reported for heterotrophic deep‐sea benthos.     

Photoacclimation of the invasive alga Caulerpa racemosa var. cylindracea to depth and daylight patterns and a putative new role for siphonaxanthin

The introduced green alga Caulerpa racemosa var. cylindracea grows along a broad depth range (from very shallow to 60 m depth) in the Mediterranean basin. In the present work, the photoacclimation capacity of this invasive variety was investigated in summer, the season of its maximum spread. Natural populations from the Gulf of Naples (Italy) were analyzed for photoresponse on two scales of light variability: a spatial scale (at three stations along a depth gradient, from 0.3 to 20 m depth) and a temporal scale (on the shallowest meadow, from sunrise to sunset). These responses were studied through pigment analysis (with HPLC), and in situ measurements of photosynthetic parameters (with a Diving‐PAM fluorometer). Electron transport rate (ETR)–irradiance curve parameters showed acclimation along environmental gradients dominated by variation in irradiance. In the shallowest plants, the lack of a midday depression in both the maximum relative ETRs and the photosynthetic efficiency at sub‐saturating irradiance (α) pointed to a maintenance of energy conversion levels despite the protective lowering of light‐harvesting efficiency revealed by the trend in F_v/F_m. On the other side, variation of photosynthetic efficiency occurred with depth and buffered the effect of decreasing light on maximum photosynthetic rates. A previously undescribed xanthophyll cycle centred on lutein‐siphonaxanthin interconversion appeared to operate in the shallowest populations in addition to the violaxanthin/antheraxanthin/zeaxanthin cycle commonly occurring in Chlorophyta; this would further enhance phototolerance of the alga. A further role of siphonaxanthin is in the acclimation to low light of deep environments as indicated by its stronger increase from the surface to the deepest station with respect to siphonein and chlorophyll b.     

Spatial distribution of phytoplankton in the White Sea in the late summer period with regard to the water structure and dynamics

The species composition and biomass of the phytoplankton, as well as the hydrophysical and hydrochemical characteristics of the water masses, were estimated for different areas of the White Sea at 24 stations for the period of August 26 to September 3, 2007. The micronutrient concentrations were lower compared to the average long-term values for the summer period. The total biomass of phytoplankton (B _t ) varied from 87 to 1914 mg C/m². For most of the studied sites, B _t was low; dinoflagellates were the most abundant species. Diatoms dominated in the areas of elevated hydrodynamic activity, i.e., in the Gorlo (vertical mixing of the water layers), in intrusions of the Gorlo water masses into the basin of the White Sea, and in the peripheral areas of the Dvina outflow current. The B _t was also the highest in the listed areas.     

增殖放流影响下东海北部海域三疣梭子蟹(Portunus trituberculatus)最大可持续产量评估

当渔业资源出现衰退时,加强资源增殖放流以养护渔业资源、提高渔业产量对于渔业资源可持续利用具有重要意义;与此同时,增殖放流的实施会对基于资源开发与管理的评估的结果产生影响。基于2001~2015年间东海北部海域三疣梭子蟹(Portunus trituberculatus)渔业数据,采用增殖剩余产量模型,对东海北部海域三疣梭子蟹的最大可持续产量(MSY)及取得MSY时所需捕捞努力量(E_MSY)和原存生物量(B_MSY)进行了评估,并与传统Schaefer模型评估结果进行了比较。结果表明,当年增殖放流量约在3×10⁶~95×10⁶尾之间时,三疣梭子蟹年产量逐渐增加, MSY在14.2×10⁴ t和14.6×10⁴ t之间, E_MSY基本在15×10⁴吨位左右。增殖放流量增加,其对应的MSY也越高,能承受的E_MSY也越高(从15×10⁴~15.4×10⁴吨位之间),相反B_MSY则减小(从188.4×10⁴ t降至186.6×10⁴ t)。与传统的Schaefer模型评估结果相比,增殖剩余产量模型由于考虑了增殖放流生物量的因素,得到了MSY和E_MSY有所增加,而B_MSY有所下降的结论。研究结果有望为该研究海域三疣梭子蟹可持续地捕捞、放流与管理提供科学依据。     

Phytoplankton absorption and pigment adaptation of a red tide in the Benguela ecosystem

Phytoplankton absorption and pigment characteristics of a red tide were investigated in coastal waters of the southern Benguela. Diagnostic indices indicated that dinoflagellates were the dominant phytoplankton group, with diatoms and small flagellates being of secondary importance. Very high biomass was observed close to the coast where chlorophyll a concentrations of up to 117 mg m^–3 were measured. Both measured (a _ph) and reconstructed pigment absorption (a _pig) displayed an increasing trend with chlorophyll a, while the package effect index (Q^* _a) decreased, indicating increased packaging with an increase in biomass. Proportioning of the total pigment absorption between 400 and 700 nm revealed that chlorophyll a accounted for 39–65% of the absorption, while photosynthetic carotenoids (15–30%) and chlorophyll cs (15–30%) were also prominent in absorbing light for photosynthesis.     

The beam attenuation to chlorophyll ratio: an optical index of phytoplankton physiology in the surface ocean?

The particulate beam attenuation coefficient (c_p) is proportional to the concentration of suspended particles in a size domain overlapping that of the phytoplankton assemblage. c_p is largely insensitive to changes in intracellular chlorophyll concentration, which varies with growth irradiance (a process termed ‘photoacclimation’). Earlier studies have shown that the ratio of c_p:chlorophyll (i.e., c_p^*) exhibits depth-dependent changes that are consistent with photoacclimation. Similar relationships may likewise be expected in the horizontal and temporal dimensions, reflecting changes in mixing depth, incident irradiance, and light attenuation. A link between c_p^* and more robust photoadaptive variables has never been explicitly tested in the field. Here we use five historical field data sets to directly compare spatial and temporal variability in c_p^* with two independent indices of photoacclimation: the light-saturated, chlorophyll-normalized photosynthetic rate, P^b_opt, and the light-saturation index, E_k. For the variety of oceanographic conditions considered, a first-order correlation emerged between c_p^* and P^b_opt or E_k. These simple empirical results suggest that a relationship exists between a bio-optical variable that can potentially be retrieved remotely (c_p^*) and physiological variables crucial for estimating primary productivity in the sea.     

Photoacclimatization in the zooxanthellae of Pocillopora verrucosa and comparison with a pelagic algal community

Photoacclimatization of zooxanthellae extracted from the coral Pocillopora verrucosa was studied through the determination of pigments, light absorption and photosynthetic parameters, for samples collected in summer and winter between 1 and 40 m on a northwestern reef of Tahiti (French Polynesia). The same measurements were also performed on phytoplanktonic samples collected at a stable oceanic site north of the island. For the zooxanthellae, the variations with depth of all the parameters were generally of small amplitude. Seasonal differences were also observed. The photosynthetic to non-photosynthetic pigments ratio was higher at depth in both seasons and was higher in winter. The intracellular concentration of chlorophyll a and photosynthetic pigments was higher in winter, as was the photosynthetic pigments/chlorophyll a ratio, whereas the non-photosynthetic pigments/chlorophyll a ratio was higher in summer. Variations in the light absorption properties were also small. The photosynthetic parameters showed limited changes with depth with the largest variations (a factor of ∼2) observed for P^B_max. The trends observed for the phytoplankton assemblage were generally of much higher amplitudes than for the zooxanthellae (e.g. for photosynthetic to non-photosynthetic pigments ratio or the saturation parameter, E_k). These results suggest that, in the very clear Polynesian waters, the amount of energy that reaches the zooxanthellae of P. verrucosa is not variable enough in the 1–40 m depth range to result in a drastic modification of the photosynthetic apparatus of the algae.     

Structure and photosynthetic properties of phytoplankton assemblages in a highly dynamic system, the Northern Adriatic Sea

The photosynthetic properties of phytoplankton populations as related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area whereas small phytoflagellates (mainly 19′-hexanoyloxyfucoxanthin, chlorophyll b, 19′-butanoyloxyfucoxanthin) occurred outside the front. The front was defined by the steep gradient in density in the surface layer separating low-salinity coastal waters from the offshore waters.Physical features of the area strongly influenced phytoplankton biomass distributions, composition and size structure. After high volumes of Po River discharge several gyres and meanders occurred in the area off the river delta in February. Decreasing river discharge and the subsequent disappearance of the gyres and the spreading dilution of the river plume was observed in March. The dynamic circulation of February resulted in high photosynthetic capacity of the abundant phytoplankton population (>3.40 mg m⁻³). In March, the slow circulation and an upper low-salinity water layer, segregated from the deeper layers, resulted in lack of renewal of this water mass. The huge phytoplankton biomass, up to 15.77 mg chl a m⁻³, became nutrient depleted and showed low photosynthetic capacity. In February, an exceptionally high P_max^B, 20.11 mg C (mg chl a)⁻¹ h⁻¹ was recorded in the Po River plume area and average P_max^B was three-fold in February as compared to the March recordings, 10.50 mg C (mg chl a)⁻¹ h⁻¹ and 3.22 mg C (mg chl a)⁻¹ h⁻¹, respectively.The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area will help to predict phytoplankton response and biomass distributions. In the frontal area, despite a considerable variability in environmental conditions, our findings have shown that the phytoplankton assemblages will compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, as a result, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.     

Phytoplankton production and adaptation in the vicinity of Pemba and Zanzibar islands,Tanzania

Phytoplankton production and physiology were investigated at six selected locations during a research cruise in early October 2007 in Tanzanian coastal waters. The dataset included photosynthesis– irradiance and active fluorescence parameters, phytoplankton absorption coefficients, and pigment concentrations. Primary production was estimated to vary over the range 0.79–1.89 g C m^?2 d^?1. Diagnostic pigments indicated that micro–nanophytoplankton comprised the communities at three stations and nano–picoplankton at the other three stations. At all stations, the populations maximised their photosynthesis in the upper water column under elevated irradiance and low nutrient conditions. Significant photosynthetic activity was also observed at depth under very low light where the communities increased their quantum yield of photochemistry and the proportion of accessory chlorophylls b and c and photosynthetic carotenoids.     

Effects of simulated benthic fluxes on phytoplankton dynamic and photosynthetic parameters in a mesocosm experiment (Bay of Brest,France)

Benthic faunal activity and density play an important role in determining the rates of benthic nutrient fluxes, which enrich the water column and contribute to phytoplankton growth. The intensity of nutrient fluxes in the Bay of Brest depends on the density of the invasive gastropod, Crepidula fornicata. In order to study the impact of benthic fluxes on phytoplankton dynamics, realistic daily nutrient inputs simulating various densities of C. fornicata were added to six enclosures during three weeks. The increase in fertilization intensity influenced the phytoplankton biomass. A succession from Chaetoceros spp. to Pseudo-nitzschia spp. and Leptocylindrus danicus was observed in all enclosures, but the dynamics of successions were different. Pseudo-nitzschia spp. was favored in the three more fertilized enclosures, while Chaetoceros spp. persisted longer in less enriched enclosures. Despite an apparent nitrogen limitation, the quantum efficiency of PSII (F_v/F_m) was high (>0.5) and stable in all enclosures. The maximal photosynthetic capacity (P^Bmax) was also invariable and oscillated around an average value of 2.23 mg C (mg Chl a)⁻¹ h⁻¹. The stability of F_v/F_m and P^Bmax observed at different nutrient input intensities demonstrates that the daily inputs maintained the physiological balance of the microalgae. The maximal light utilization efficiency (α) and the light saturation parameter (E_k) were also quite stable after day 8, which reveals that photosynthetic parameters were driven by growth constraints due to nutrient availability and not by incident light or species successions. We suggest that our results correspond to an “E_k independent variation” regulation. We propose that such regulation of photosynthetic parameters appears when there are frequent nutrient additions which do not allow replete nutrient conditions to be reached but lead to physiological equilibrium.     

Productivity of dinoflagellate blooms on the west coast of South Africa,as measured by natural fluorescence

The biomass and productivity of phytoplankton populations inshore on the west coast of South Africa were investigated towards the end of the upwelling season, a period when high-biomass dinoflagellate blooms are common. Productivity was estimated from natural fluorescence measurements (P_NF ), using photosynthesis (P) v. irradiance (E) relationships (P_E ) and by means of the in situ ¹⁴C-method (P_C ) A linear regression of P_NF productivity against P_C and P_E productivities yielded a slope of 0.911 and an r ₂ of 0.83 (n = 41). Physical and biological variability was high inshore, reflecting alternating periods of upwelling and quiescence. Mean chlorophyll inshore (within a 12 m water column) ranged from 0.7 to 57.8 (mean = 8.9) mg·m^&minus3, mean P_NF productivity ranged from 8.4 to 51.0 (mean = 24.6) mgC·m^?3·h^?1 and daily integral P_NF productivity from 0.8 to 4.8 (mean = 2.3) gC·m^?2·day^?l. Transects sampled during active and relaxation phases of upwelling had different chlorophyll distributions. High chlorophyll concentrations (sometimes >50 mg·m^?3) were associated with surface blooms within the region of the upwelling front. Estimates of daily water-column P_NF productivity within these frontal blooms ranged from 4.0 to 5.6 gC·m^?2·day^?1. With relaxation of wind stress, blooms dominated by dinoflagellates flooded shorewards and often formed red tides. Chlorophyll concentrations of > 175 mg·m^?3 and productivity rates > 500 mgC·m^?3·h^?1 and 12 gC·m^?2·day^?1 were measured during a particularly intense red tide. Offshore, the water column was highly stratified with a well-defined subsurface chlorophyll maximum layer within the pycnocline region. Estimates of daily water-column P_NF productivity ranged from 2.4 to 4.0 gC·m^?2·day^?1 offshore. The high productivity of shelf waters on the West Coast in late summer can be ascribed largely to dinoflagellate populations and their success in both upwelling systems and stratified conditions.     

Evidence of microphytobenthic roles on coastal shallow water of the Seto Inland Sea,Japan

This study describes the temporal variation of microphytobenthic biomass and its controlling factors, as well as the impact of microphytobenthic activities on coastal shallow sediment in the eastern Seto Inland Sea, Japan. The chlorophyll a (Chl a), phaeopigments and sedimentary biophilic element (C, N, P and Si) contents in surface sediments, as well as nutrient concentrations at the sediment-water interface (overlying water and pore water) were observed monthly during November 2003 to May 2005 at one site in Shido Bay (water depth ca. 7 m) and at one site in Harima-Nada (35 m). No correlation was observed between the sedimentary biophilic elements and other parameters. The maximum chlorophyll a content of 48.2 μg g^–1 was found in surface sediments under the photon flux reaching the seafloor of 537 μmol photon m^–2 s^–1 during the winter period when water transparency was the highest at station S (Shido Bay). Our results suggest that higher chlorophyll a content in surface sediment was due to the fresh microphytobenthic biomass (mainly benthic diatom). We also found a significant negative correlation between Chl a and inorganic nutrients in pore water during the low temperature period, especially silicic acid. This result suggests that the silicic acid was assimilated largely during the increase of microphytobenthic biomass in surface sediment. This study suggests that the microphytobenthic roles may have a great effect on nutrient budgets during the large supply of irradiance (winter periods) for their photosynthetic growth in shallow coastal waters.     

Phytoplankton production in the Tagus estuary (Portugal)

Biomass and phytoplankton photosynthetic response were studied in the lower Tagus estuary weekly, and related to environmental conditions in February, March and April 1994. The Photosynthesis-Irradiance (P₋^BI) relation was studied based on the light-saturated photosynthesis rate (P^Bm) and the light-limited initial slope (a^B). The nutrient concentrations observed were high enough to be considered as not limiting phytoplankton growth. Tagus estuary phytoplankton seems, to a certain extent, adapted to high turbid conditions, being able to utilize the low light levels more efficiently, which was translated by high values of a^B [0.10–0.20 mg C (mg Chl a)⁻¹ h⁻¹ (W m⁻²)⁻¹]; however, light seems to limit phytoplankton production in the water column.