Role of mesoscale eddies in the sustenance of high biological productivity in North Eastern Arabian Sea during the winter-spring transition period

Convective mixing, mesoscale eddies and regenerated production sustain an above-average biological productivity in the North East Arabian Sea (NEAS) during the winter-spring transition period. Satellite-derived long-term data sets on Chlorophyll-a (Chl-a), Sea Surface Height Anomaly (SSHA), Sea Surface Temperature (SST) and Okubo-Weiss parameterization show existence of number of mesoscale eddies, propagating and non-propagating, that contribute to the regional production. The dominance of Eddy Kinetic Energy (EKE) over the Available Potential Energy (APE) in the core depth and the diameter (120km) of the observed eddy being wider than the Rossby Radius of Deformation (RRD, 55 km), it is suggested that the baroclinic instability is a possible mechanism for the eddy formation. Spatial variation in APE and its influences on the regional dynamics, including chemical and biological response are explained. In the non-eddy areas, where convective mixing is active, diatoms (96.74%) dominated than dinoflagellates (3.14%), and the Chl-a in the Cold Core Eddy (CCE) were two to three folds higher to non-eddy regions. The abundance increased from core (58,152 cells L^-1) to periphery (5.95 × 10⁵ cells L^-1) where the water column is less dynamic. Extensive blooms of the dinoflagellate green Noctiluca (N. scintillans) contribute to the very high cell density in the periphery of the CCE, where the currents were comparatively weak, and water column was more stable. Active mixing is associated with diatom dominance, followed by Noctiluca when the mixing slackens, making use of the available nutrients and supported by regenerated production. The bloom dynamics is explained for pre-bloom, bloom and post-bloom conditions with measurements on nutrients and plankton assemblages. The Noctiluca bloom (mid-March) is succeeded by Trichodesmium (April-May), in the stratified nutrient depleted, abundant light environment and propagated southwards. Observed increasing trends in the SSHA over the period indicate strengthening of stratification and hence altered production patterns in the NEAS.

Responses of marine zooplankton indicators after five years of a dam rupture in the Doce River, Southeastern Brazil

Since November of 2015, when ore tailings from a dam rupture reached the Atlantic Ocean, researchers are trying to assess the degree of impact across the Doce River and adjacent coastal area. This study aims to use the zooplankton dynamics as a tool to evaluate the environmental impact in the coastal region, five years after the rupture, during periods of low and high river flow. Doce River flow varied from 49 to 5179 m³/s and structured the zooplankton community between periods of low and high river flow, but salinity and chlorophyll-a had stronger correlation with depth (r = 0.40 and - 0.40 respectively) than with the Doce River discharge variation along the sampling period (r < 0.2). On the other hand, inorganic particles in the water and total metal concentration (dissolved + particulate), used as tracers of the iron enriched tailing (Al, Cd, Cr, Cu, Fe, V), were correlated with fluvial discharge and showed to be the main factor driving the zooplankton community dynamics. For assessing the degree of environmental impact, we tested the ecological indexes for the zooplankton community. Margalef Richness, Pielou Evenness and Shannon-Wiener Diversity varied from 2.52, 0.40 and 1.39 (all registered during high river flow period) to 9.02, 0.85 and 3.44 (all registered during low river flow period), respectively. Along with those community indicators, we evaluated the response of representative taxonomical genera such as Paracalanus, Oikopleura and Temora, regarding the Doce River flow, and found population patterns that established a baseline for future monitoring in the region. Our results showed that the zooplankton community is more fragile when the river discharge is stronger, and this pattern is confirmed by all indicators tested.

Student's tutorial on bloom hypotheses in the context of phytoplankton annual cycles

Phytoplankton blooms are elements in repeating annual cycles of phytoplankton biomass and they have significant ecological and biogeochemical consequences. Temporal changes in phytoplankton biomass are governed by complex predator-prey interactions and physically driven variations in upper water column growth conditions (light, nutrient, and temperature). Understanding these dependencies is fundamental to assess future change in bloom frequency, duration, and magnitude and thus represents a quintessential challenge in global change biology. A variety of contrasting hypotheses have emerged in the literature to explain phytoplankton blooms, but over time the basic tenets of these hypotheses have become unclear. Here, we provide a "tutorial" on the development of these concepts and the fundamental elements distinguishing each hypothesis. The intent of this tutorial is to provide a useful background and set of tools for reading the bloom literature and to give some suggestions for future studies. Our tutorial is written for "students" at all stages of their career. We hope it is equally useful and interesting to those with only a cursory interest in blooms as those deeply immersed in the challenge of understanding the temporal dynamics of phytoplankton biomass and predicting its future change.

Seasonality of North Atlantic phytoplankton from space: impact of environmental forcing on a changing phenology (1998-2012)

Seasonal pulses of phytoplankton drive seasonal cycles of carbon fixation and particle sedimentation, and might condition recruitment success in many exploited species. Taking advantage of long-term series of remotely sensed chlorophyll a (1998-2012), we analyzed changes in phytoplankton seasonality in the North Atlantic Ocean. Phytoplankton phenology was analyzed based on a probabilistic characterization of bloom incidence. This approach allowed us to detect changes in the prevalence of different seasonal cycles and, at the same time, to estimate bloom timing and magnitude taking into account uncertainty in bloom detection. Deviations between different sensors stressed the importance of a prolonged overlap between successive missions to ensure a correct assessment of phenological changes, as well as the advantage of semi-analytical chlorophyll algorithms over empirical ones to reduce biases. Earlier and more intense blooms were detected in the subpolar Atlantic, while advanced blooms of less magnitude were common in the Subtropical gyre. In the temperate North Atlantic, spring blooms advanced their timing and decreased in magnitude, whereas fall blooms delayed and increased their intensity. At the same time, the prevalence of locations with a single autumn/winter bloom or with a bimodal seasonal cycle increased, in consonance with a poleward expansion of subtropical conditions. Changes in bloom timing and magnitude presented a clear signature of environmental factors, especially wind forcing, although changes on incident photosynthetically active radiation and sea surface temperature were also important depending on latitude. Trends in bloom magnitude matched changes in mean chlorophyll a during the study period, suggesting that seasonal peaks drive long-term trends in chlorophyll a concentration. Our results link changes in North Atlantic climate with recent trends in the phenology of phytoplankton, suggesting an intensification of these impacts in the near future.

Resurrecting the ecological underpinnings of ocean plankton blooms

Nutrient and light conditions control phytoplankton division rates in the surface ocean and, it is commonly believed, dictate when and where high concentrations, or blooms, of plankton occur. Yet after a century of investigation, rates of phytoplankton biomass accumulation show no correlation with cell division rates. Consequently, factors controlling plankton blooms remain highly controversial. In this review, we endorse the view that blooms are not governed by abiotic factors controlling cell division, but rather reflect subtle ecosystem imbalances instigated by climate forcings or food-web shifts. The annual global procession of ocean plankton blooms thus represents a report on the recent history of predator-prey interactions modulated by physical processes that, almost coincidentally, also control surface nutrient inputs.

Abandoning Sverdrup's Critical Depth Hypothesis on phytoplankton blooms

The Critical Depth Hypothesis formalized by Sverdrup in 1953 posits that vernal phytoplankton blooms occur when surface mixing shoals to a depth shallower than a critical depth horizon defining the point where phytoplankton growth exceeds losses. This hypothesis has since served as a cornerstone in plankton ecology and reflects the very common assumption that blooms are caused by enhanced growth rates in response to improved light, temperature, and stratification conditions, not simply correlated with them. Here, a nine-year satellite record of phytoplankton biomass in the subarctic Atlantic is used to reevaluate seasonal plankton dynamics. Results show that (1) bloom initiation occurs in the winter when mixed layer depths are maximum, not in the spring, (2) coupling between phytoplankton growth (micro) and losses increases during spring stratification, rather than decreases, (3) maxima in net population growth rates (r) are as likely to occur in midwinter as in spring, and (4) r is generally inversely related to micro. These results are incompatible with the Critical Depth Hypothesis as a functional framework for understanding bloom dynamics. In its place, a "Dilution Recoupling Hypothesis" is described that focuses on the balance between phytoplankton growth and grazing, and the seasonally varying physical processes influencing this balance. This revised view derives from fundamental concepts applied during field dilution experiments, builds upon earlier modeling results, and is compatible with observed phytoplankton blooms in the absence of spring mixed layer shoaling.

The role of virus infection in a simple phytoplankton zooplankton system

Many planktonic species show spectacular bursts ("blooms") in population density. Though viral infections are known to cause behavioural and other changes in phytoplankton and other aquatic species, yet their role in regulating the phytoplankton population is still far from being understood. To study the role of viral diseases in the planktonic species, we model the phytoplankton-zooplankton system as a prey-predator system. Here the prey (phytoplankton) species is infected with a viral disease that divides the prey population into susceptible and infected classes, with the infected prey being more vulnerable to predation by the predator (zooplankton). The dynamical behaviour of the system is investigated from the point of view of stability and persistence both analytically and numerically. The model shows that infection can be sustained only above a threshold of force of infection, and, there exists a range in the infection rate where this system shows "bloom"-like stable limit cycle oscillations. The time series of natural "blooms" with different types of irregular oscillations can arise in this model simply from a biologically realistic feature, i.e., by the random variation of the epidemiological parameter (rate of infection) in the infected prey population. The difference in mean strength of infection alone can lead to the different types of patterns observed in natural planktonic blooms.

The role of phytoplankton photosynthesis in global biogeochemical cycles

Phytoplankton biomass in the world's oceans amounts to only ∽1-2% of the total global plant carbon, yet these organisms fix between 30 and 50 billion metric tons of carbon annually, which is about 40% of the total. On geological time scales there is profound evidence of the importance of phytoplankton photosynthesis in biogeochemical cycles. It is generally assumed that present phytoplankton productivity is in a quasi steady-state (on the time scale of decades). However, in a global context, the stability of oceanic photosynthetic processes is dependent on the physical circulation of the upper ocean and is therefore strongly influenced by the atmosphere. The net flux of atmospheric radiation is critical to determining the depth of the upper mixed layer and the vertical fluxes of nutrients. These latter two parameters are keys to determining the intensity, and spatial and temporal distributions of phytoplankton blooms. Atmospheric radiation budgets are not in steady-state. Driven largely by anthropogenic activities in the 20th century, increased levels of IR- absorbing gases such as CO2, CH4 and CFC's and NOx will potentially increase atmospheric temperatures on a global scale. The atmospheric radiation budget can affect phytoplankton photosynthesis directly and indirectly. Increased temperature differences between the continents and oceans have been implicated in higher wind stresses at the ocean margins. Increased wind speeds can lead to higher nutrient fluxes. Throughout most of the central oceans, nitrate concentrations are sub-micromolar and there is strong evidence that the quantum efficiency of Photosystem II is impaired by nutrient stress. Higher nutrient fluxes would lead to both an increase in phytoplankton biomass and higher biomass-specific rates of carbon fixation. However, in the center of the ocean gyres, increased radiative heating could reduce the vertical flux of nutrients to the euphotic zone, and hence lead to a reduction in phytoplankton carbon fixation. Increased desertification in terrestrial ecosystems can lead to increased aeolean loadings of essential micronutrients, such as iron. An increased flux of aeolean micronutrients could fertilize nutrient-replete areas of the open ocean with limiting trace elements, thereby stimulating photosynthetic rates. The factors which limit phytoplankton biomass and photosynthesis are discussed and examined with regard to potential changes in the Earth climate system which can lead the oceans away from steady-state. While it is difficult to confidently deduce changes in either phytoplankton biomass or photosynthetic rates on decadal time scales, time-series analysis of ocean transparency data suggest long-term trends have occurred in the North Pacific Ocean in the 20th century. However, calculations of net carbon uptake by the oceans resulting from phytoplankton photosynthesis suggest that without a supply of nutrients external to the ocean, carbon fixation in the open ocean is not presently a significant sink for excess atmospheric CO2.