Magnitude Of Phytoplankton Blooms Research Articles

A simple metric for predicting the timing of river phytoplankton blooms

AbstractIn rivers, phytoplankton populations are continuously exported by unidirectional, advective flow. Both transport and growth conditions determine periods of excess phytoplankton growth, or blooms, in a given reach. Phytoplankton abundance, however, has mainly been compared to the state of either growth or transport conditions alone rather than in tandem. Previous studies have not yielded consistent driver–response relationships, limiting our ability to predict the timing of riverine phytoplankton blooms based on environmental factors. Here, we derive a simple joint metric that combines the state of growth and transport conditions, specifically the ratio of temperature and discharge (). We then compare the metric to biomass abundance data (daily, sensor‐based chlorophyll a [chl a] data) from a mid‐sized Great Plains river (the Kansas). was an excellent predictor of low to high biomass, outperforming either variable alone. However, it could not differentiate between very high biomass values, values well above the biomass threshold designating bloom conditions. Our findings of reduced performance at the highest values of T/Q indicated that could predict the occurrence (timing) but not magnitude of phytoplankton blooms, and we used T/Q to correctly predict 71% of days when bloom conditions occurred. Analyzing chl a abundance with also revealed likely switching from transport and temperature to nutrient control. offers a simple tool for (1) predicting the timing of river phytoplankton blooms, (2) forecasting how river ecosystems will respond to surrounding environmental changes, and (3) determining which environmental factors shape phytoplankton blooms at specific locations along a river.

Remote silicate supply regulates spring phytoplankton bloom magnitude in the Gulf of Maine

AbstractSpring phytoplankton blooms in the Gulf of Maine (GoM) are sensitive to climate‐related local and remote forcing. Nutrient supply through the slope water intrusion has been viewed as critical in regulating the GoM spring blooms, with an assumption that nitrogen is the primary limiting nutrient. In recent years, this paradigm has been challenged, with silicate being recognized as another potential limiting nutrient, but the source of silicate and its associated water mass remain difficult to be determined. In this study, a time series of spring bloom magnitude was constructed using a self‐organizing map algorithm, and then correlated with the fluctuation of water composition in the deep Northeast Channel. The results reveal the importance of silicate supply from previously less‐recognized deep Scotian Shelf Water inflow. This study offers a new hypothesis for spring bloom regulation, providing a better understanding of mechanisms controlling the spring bloom magnitude in the GoM.

Associations between timing and magnitude of spring blooms and zooplankton dynamics in the southwestern Barents Sea

During the past decades, many high-latitude marine systems have experienced a strong warming trend with poorly understood consequences for trophic coupling and ecosystem functioning. A key knowledge gap is how timing and magnitude of phytoplankton blooms influence higher trophic levels. We investigated associations between timing and magnitude of phytoplankton blooms and dynamics of 3 size fractions of mesozooplankton from 1998 to 2019. The study focused on the southwestern Barents Sea, an Arctic shelf sea area that is dominated by relatively warm Atlantic waters and which remains ice-free year-round. Results showed that an early spring bloom (late April to early May) was associated with high biomass of medium-sized (1-2 mm) zooplankton in areas ‘downstream’ of the phytoplankton bloom, along the prevailing currents. Conversely, a late spring bloom was associated with high biomass of small-sized (0.180-1 mm) zooplankton, with no spatial shift. High peak magnitude of the bloom (>5 mg chl a m-3) was associated with low zooplankton biomass, suggesting either top-down control or that the zooplankton utilized intense and presumably short blooms inefficiently. For small- and large-sized (>2 mm) zooplankton, the relationship was nonlinear, as zooplankton biomass was also low when bloom peak magnitude was very low (<4 mg chl a m-3). Our findings imply that if phytoplankton blooms in the region occur earlier, this will increase the biomass of medium-sized zooplankton that are important prey for planktivorous fishes. Moreover, our study highlights that increased biomass of phytoplankton does not necessarily translate into increased zooplankton biomass.

Effect of environmental factors and density-dependence on somatic growth of Eastern Georges Bank haddock (Melanogrammus aeglefinus)

Haddock (Melanogrammus aeglefinus) is an important commercial fish species in the Northwest Atlantic, with the Eastern Georges Bank population a transboundary resources utilized by both Canada and the United States. With the dramatic increase recruitment of this population in the last decade, in particular concentrations of fish on the eastern portion of the Bank, there has been a precipitous decrease in somatic growth, raising concerns for managers and stakeholders. In this study, the effect of environmental factors and density-dependence on somatic growth of eastern Georges Bank haddock was studied using a 2-step Generalized Additive Model (GAM) regression approach. The biotic and abiotic factors examined in the regression analysis included haddock density, sampling location, local sea surface and bottom temperature, salinity and short time series of phytoplankton bloom statistics. The relative importance of each covariate in the best selected model was investigated using the nested GAMs. We found that density-dependence explained most of the variation in haddock growth, where the impact of temperature varied by season. Though of less model importance, annual winter salinity and phytoplankton bloom magnitude (lagged by one year) also contributed the growth model fits. These relationships have the immediate application of contextualizing the observed change in haddock growth, but may also inform various aspects of the stock assessment process.

Declining reproductive success in the Gulf of St. Lawrence's humpback whales (Megaptera novaeangliae) reflects ecosystem shifts on their feeding grounds.

Climate change has resulted in physical and biological changes in the world's oceans. How the effects of these changes are buffered by top predator populations, and therefore how much plasticity lies at the highest trophic levels, are largely unknown. Here endocrine profiling, longitudinal observations of known individuals over 15years between 2004 and 2018, and environmental data are combined to examine how the reproductive success of a top marine predator is being affected by ecosystem change. The Gulf of St. Lawrence, Canada, is a major summer feeding ground for humpback whales (Megaptera novaeangliae) in the North Atlantic. Blubber biopsy samples (n=185) of female humpback whales were used to investigate variation in pregnancy rates through the quantification of progesterone. Annual pregnancy rates showed considerable variability, with no overall change detected over the study. However, a total of 457 photo-identified adult female sightings records with/without calves were collated, and showed that annual calving rates declined significantly. The probability of observing cow-calf pairs was related to favourable environmental conditions in the previous year; measured by herring spawning stock biomass, Calanus spp. abundance, overall copepod abundance and phytoplankton bloom magnitude. Approximately 39% of identified pregnancies were unsuccessful over the 15years, and the average annual pregnancy rate was higher than the average annual calving rate at ~37% and ~23% respectively. Together, these data suggest that the declines in reproductive success could be, at least in part, the result of females being unable to accumulate the energy reserves necessary to maintain pregnancy and/or meet the energetic demands of lactation in years of poorer prey availability rather than solely an inability to become pregnant. The decline in calving rates over a period of major environmental variability may suggest that this population has limited resilience to such ecosystem change.

Temperature-related timing of the spring bloom and match between phytoplankton and zooplankton

ABSTRACT Global warming is causing changes in the food web structure and seasonal plankton dynamics. The Baltic Sea is one of the fastest-warming sea areas and warming consequently affects the timing and magnitude of phytoplankton blooms. Based on available Chlorophyll a data from nine years between 1979 and 2018, from the entrance to the Gulf of Finland, we studied the timing of the phytoplankton spring bloom in relation to spring seawater temperature. We found the peak of the bloom to occur earlier in years with higher spring seawater temperature. In warmer years, there was also a shorter time lag between phytoplankton and zooplankton biomass peaks. In addition, it seems as the spring bloom total biomass has decreased with time, with more extensive summer blooms during later years, in line with the general trend observed in the Baltic Sea. The spring bloom has traditionally been considered the most important part of the season, but we argue that the whole growth season should be investigated since summer blooms appear to increase with warming.

Microbial Competition in the Subpolar Southern Ocean: An Fe–C Co-limitation Experiment

Iron (Fe) is a paradox in the modern ocean – it is central to many life-critical enzymes but is scarce across most surface waters. The high cellular demand and low bioavailability of Fe likely puts selective pressure on marine microorganisms. Previous observations suggest that heterotrophic bacteria are outcompeted by small diatoms for Fe supply in the subantarctic zone of Southern Ocean, thereby challenging the idea of heterotrophic bacteria being more competitive than phytoplankton in the access to this trace metal. To test this hypothesis, incubation experiments were carried out at the Southern Ocean Time Series site (March–April 2016). We investigated (a) whether dissolved organic carbon (DOC), dissolved Fe, or both limit the growth of heterotrophic bacteria and, (b) if the presence of potential competitors has consequences on the bacterial Fe acquisition. We observed a pronounced increase in both bulk and cell-specific bacterial production in response to single (+C) and combined (+Fe+C) additions, but no changes in these rates when only Fe was added (+Fe). Moreover, we found that +Fe+C additions promoted increases in cell-specific bacterial Fe uptake rates, and these increases were particularly pronounced (by 13-fold) when phytoplankton were excluded from the incubations. These results suggest that auto- and heterotrophs could compete for Fe when DOC limitation of bacterial growth is alleviated. Such interactions between primary producers and nutrient-recyclers are unexpected drivers for the duration and magnitude of phytoplankton blooms in the Southern Ocean.

Spatial heterogeneity of seasonal phytoplankton blooms in a marginal sea: physical drivers and biological responses

Abstract Satellite and in situ observations are used in this study to examine spatial heterogeneity in the timing and magnitude of phytoplankton blooms in relation to local and remote physical processes in the Yellow Sea (YS), a marginal sea in the northwestern Pacific Ocean. Satellite ocean colour data reveal that annual chlorophyll maximums vary significantly in both timing and magnitude over different subregions of the YS. Strong summer blooms were found off estuary regions, and widespread spring blooms were found in the central trough. Localized autumn and winter peaks were found in small patches around Jeju Island and in nearshore regions. A statistical analysis of in situ measurements of the western YS suggests that variability in hydrographic properties could explain most of the spatial heterogeneity observed in both bloom timing and magnitude. The spatial heterogeneity of hydrographic properties, such as stratification and nutrient availability, are controlled by a suite of physical forcings, including the extent of the YS Cold Water Mass, river discharge, warm slope water intrusion, and seasonal warming/cooling. Our results imply that the spatial heterogeneity of marginal seas must be carefully considered when assessing phytoplankton responses in the context of climate change, because of the complexity of underlying mechanisms.

Interannual Improvement in Sea Lettuce Blooms in an Agricultural Catchment

Riverine nutrient loading from agriculture is one of the most prominent pressures in the second cycle of river basin management planning for the European Union (EU) Water Framework Directive (WFD). Better farmyard nutrient management planning is the measure most likely to reduce agricultural nutrient loading to catchment watercourses and coastal receiving waters. The adjoining Argideen Estuary and Courtmacsherry Bay in the south west of Ireland drain a 150km2 catchment comprising mainly agricultural land. The receiving waters were allocated Poor ecological status under the WFD at the most recent appraisal. Subhourly water quality monitoring in the Timoleague River has been carried out by the Teagasc Agricultural Catchments Programme to track the changes in nutrient loading to the Argideen Estuary in response to improved farming practice. A biophysical model of the adjoining Argideen Estuary and Courtmacsherry Bay was calibrated and Six nutrient load scenarios were simulated to determine their impact upon macroalgae and phytoplankton bloom magnitude. In addition, nutrient flow-load relationships were derived for summers 2010 and 2016 to elucidate the improvements induced by better catchment management practice. Flow-load relationships for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) for each year were applied to the flow data for the other year, to query the outcome if 1) there had been no change in farm management practice between 2010 and 2016, or 2) the improved farm management practice in place by 2016 had been implemented by 2010. The difference between expediting and delaying improvement in farm nutrient management practice was a 5% increase in DIN loading and a 233% increase in DIP load. Application of this higher estimated load to the calibrated bio-physical model projected an increase in 2016 Ulva bloom magnitude from 381t to 1,391t. Although phosphorus retention within the catchment has improved in recent years, with an attendant improvement in Ulva bloom magnitudes, flow connectivity in the catchment still facilitates a higher phosphorus transfer during large rainfall events. A high amount of phosphorus is stored within the catchment, while point source pressures continue to contribute to phosphorus transfer to streams during periods of low flow.

Drought and management effects on biophysicochemistry in a rapidly-flushed reservoir

ABSTRACTBellinger BJ, Richter A, Porras A, Davis SL. 2017. Drought and management effects on biophysicochemistry in a rapidly flushed reservoir. Lake Reserv Manage. 34:182—198.During a recent drought period, a municipal reservoir was affected by increased duration and magnitude of cyanobacterial blooms, deep water hypoxia, and abundant Hydrilla verticillata growth. We analyzed over 20 yr of water quality data from sources monitoring Lake Austin, Texas, USA, to relate changing hydraulic pulsing rates during periods of water surplus and deficit to system biophysicochemistry. During the drought period, average daily discharges through the reservoir system were significantly reduced to conserve water supply. In response, growing season hydraulic residence times in Lake Austin, typically a run-of-river system with residence of about 10 d, increased 2– to 8–fold. Nutrient loadings to Lake Austin decreased during low discharge, but only total phosphorus (TP) concentrations declined appreciably in the reservoir. Late summer lower flows across years created epilimnetic conditions conducive to cyanobacteria blooms because of stable stratification, water temperatures exceeding 25 C, and nitrate concentration minima with TP and ammonia maxima. Drought–induced discharge reductions promoted earlier onset of warm, stratified waters, and lower hypolimnetic oxygen concentrations. The physicochemical changes coupled with longer hydraulic residence altered the timing of algal bloom initiation, duration, and peak, and quadrupled the probability of cyanobacterial bloom occurrence. The largest cyanobacteria blooms were observed when growing season discharges were consistently below 20 m3/s (i.e., hydraulic residence time >2 weeks). Concomitantly, the nonnative macrophyte hydrilla rapidly spread in the warm, reduced flow conditions. By linking flushing rates to reservoir stratification and potential phytoplankton bloom magnitude, our results will allow managers to anticipate summer water quality conditions.

Delineating environmental control of phytoplankton biomass and phenology in the Southern Ocean

AbstractThe Southern Ocean (SO), an area highly sensitive to climate change, is currently experiencing rapid warming and freshening. Such drastic physical changes might significantly alter the SO's biological pump. For more accurate predictions of the possible evolution of this pump, a better understanding of the environmental factors controlling SO phytoplankton dynamics is needed. Here we present a satellite‐based study deciphering the complex environmental control of phytoplankton biomass (PB) and phenology (PH; timing and magnitude of phytoplankton blooms) in the SO. We reveal that PH and PB are mostly organized in the SO at two scales: a large latitudinal scale and a regional scale. Latitudinally, a clear gradient in the timing of bloom occurrence appears tightly linked to the seasonal cycle in irradiance, with some exceptions in specific light‐limited regimes (i.e., well‐mixed areas). Superimposed on this latitudinal scale, zonal asymmetries, up to 3 orders of magnitude, in regional‐scale PB are mainly driven by local advective and iron supply processes. These findings provide a global understanding of PB and PH in the SO, which is of fundamental interest for identifying and explaining ongoing changes as well as predicting future changes in the SO biological pump.

Light availability rather than Fe controls the magnitude of massive phytoplankton bloom in the Amundsen Sea polynyas, Antarctica

AbstractAmundsen Sea polynyas are among the most productive, yet climate‐sensitive ecosystems in the Southern Ocean and host massive annual phytoplankton blooms. These blooms are believed to be controlled by iron fluxes from melting ice and icebergs and by intrusion of nutrient‐rich Circumpolar Deep Water, however the interplay between iron effects and other controls, such as light availability, has not yet been quantified. Here, we examine phytoplankton photophysiology in relation to Fe stress and physical forcing in two largest polynyas, Amundsen Sea Polynya (ASP) and Pine Island Polynya (PIP), using the combination of high‐resolution variable fluorescence measurements, fluorescence lifetime analysis, photosynthetic rates, and Fe‐enrichment incubations. These analyses revealed strong Fe stress in the ASP, whereas the PIP showed virtually no signatures of Fe limitation. In spite of enhanced iron availability in the PIP, chlorophyll biomass remained ∼ 30–50% lower than in the Fe‐stressed ASP. This apparent paradox would not have been observed if iron were the main control of phytoplankton bloom in the Amundsen Sea. Long‐term satellite‐based climatology records revealed that the ASP is exposed to significantly higher solar irradiance levels throughout the summer season, as compared to the PIP region, suggesting that light availability controls the magnitude of phytoplankton blooms in the Amundsen Sea. Our data suggests that higher Fe availability (e.g., due to higher melting rates of ice sheets) would not necessarily increase primary productivity in this region. Furthermore, stronger wind‐driven vertical mixing in expanding ice‐free areas may lead to reduction in light availability and productivity in the future.

Declining riverine sediment input impact on spring phytoplankton bloom off the Yangtze River Estuary from 17-year satellite observation

Off the Yangtze River Estuary and its adjacent waters (the YRE) are one of the fastest changing regions in the world in terms of the effects of anthropogenic disturbance. Here we address quantitative analysis whether reducing river to sea sediment may cause declining water turbidity then a better light available condition for the algal growth, therefor increasing phytoplankton bloom magnitude in the YRE in the bloom season. An area of high phytoplankton productivity zone is estimated by theoretical and satellite data analysis, which matches well with the spatial distribution of accumulative times of the reported algal bloom events at the decadal time scale. We present 17-year (1998–2014) satellite and hydrological data to reveal an increasing trend in Chlorophyll-a concentration (Chl-a) in the spring bloom season (May to June), which has strong correlation with the decreasing in the sediment discharge from the Yangtze river to the East China Sea. Changes in Chl-a and the sediment load are inversely related in terms of both temporal variation and their corresponding magnitudes (R2=0.38, p=0.008, n=17). Furtherly, this relationship is not sensitivity to one-year time lag analysis. On the other hand, euphotic depth in the bloom period shows no significant change, which reflects a balance between the increasing phytoplankton biomass enhancing water turbidity and declining riverine sediment decreasing turbidity. Finally, a stepwise multiple linear regression is used to determine which of the five relatively independent environmental variables most significantly contribute to the interannual variability of the bloom magnitude. The most significant effect (p=0.00007) is also found in the riverine sediment load. Therefor, our results suggest that anthropogenic derived riverine sediment change has been significantly impacted spring phytoplankton production in the YRE.

Shifting Trends in Bimodal Phytoplankton Blooms in the North Pacific and North Atlantic Oceans From Space With the Holo-Hilbert Spectral Analysis

Merged satellite ocean color data were used to examine trends in the timing and magnitude of phytoplankton blooms. Special emphasis was placed on the peak shift of spring and autumn/winter blooms in the North Pacific and North Atlantic Oceans with bimodal seasonal cycles. Ensemble empirical mode decomposition and Holo-Hilbert spectral analysis were combined to extract seasonal signals and investigate their modulation at multiple time-scales. In the temperate North Atlantic Ocean, earlier and decreasing spring blooms were detected with delayed and increased autumn blooms. The temperate North Pacific Ocean presented delayed and increased spring and autumn blooms, with the delay in fall blooms was larger than spring ones. The separation between two bloom peaks was increasing in both temperate North Atlantic and Pacific regions. The intrinsic variation in bloom timing and magnitude in these selected regions could be clearly extracted by ensemble empirical mode decomposition. Further Holo-Hilbert spectral results showed that changes in the annual cycle and bloom characteristics were modulated by interannual variability. These results suggest the critical role of interannual variability in the modulation of phytoplankton seasonality.

Relationship between magnitude of phytoplankton blooms and rainfall in a hyper-eutrophic lagoon: A continuous monitoring approach

To evaluate the effect of rainfall intensity on phytoplankton blooms, a continuous monitoring system was deployed during 2015 in a hyper-eutrophic lagoon in Taiwan. Intensive rainfall occurs during the wet summer months, from May to September. Salinity in the lagoon was found to decrease with increasing intensity of rainfall. The magnitude of phytoplankton blooms also increased linearly with increasing rainfall intensity. The chlorophyll a concentration rose by an order of magnitude during the heaviest rainfall. Blooms may be fueled by nutrient enrichment caused by drainage or run-off water from surrounding areas that is channeled into the lagoon during rainfall events. During bloom periods, the rates of net primary production and ecosystem respiration were high. However, this ecosystem was autotrophic for most of the year. As extreme rainfall is predicted to increase, the results of this study imply that the frequency and magnitude of phytoplankton blooms may increase in the future.

Physiological and ecological drivers of early spring blooms of a coastal phytoplankter.

Climate affects the timing and magnitude of phytoplankton blooms that fuel marine food webs and influence global biogeochemical cycles. Changes in bloom timing have been detected in some cases, but the underlying mechanisms remain elusive, contributing to uncertainty in long-term predictions of climate change impacts. Here we describe a 13-year hourly time series from the New England shelf of data on the coastal phytoplankter Synechococcus, during which the timing of its spring bloom varied by 4 weeks. We show that multiyear trends are due to temperature-induced changes in cell division rate, with earlier blooms driven by warmer spring water temperatures. Synechococcus loss rates shift in tandem with division rates, suggesting a balance between growth and loss that has persisted despite phenological shifts and environmental change.

The use of mussels for mitigating the noxious effect of phytoplankton spring blooms on farmed fish

The possibility of using the natural biofiltration power of blue mussels (Mytilus edulis, 0.37±0.08gind−1 dry weight) to dampen the potential detrimental effect of phytoplankton blooms on juvenile farmed sea bass (Dicentrarchus labrax) was tested in a fish farm during a 35-day mesocosm experiment. Mussel effective clearance rates averaged 41.15±14.19m3h−1 and led to a 6.3–13.1-fold reduction of the phytoplankton abundance as well as comparable decreases in chlorophyll a and turbidity. This improvement in seawater quality significantly enhanced fish physiological performances: weight-based growth rates were significantly higher (2.87±0.43%d−1) compared to control exposed to non-filtered (bulk) seawater (2.55±0.44%d−1). The same observation holds for the Fulton condition index and the metabolic activity (RNA:DNA ratio). For fish reared in bulk seawater, diatoms embedded in gills (Rhizosolenia imbricata, Thalassiosira sp.) and mucus overproduction indicated a stress (i.e. mechanical damages) induced by phytoplankton exposure which, in turn, may have affected fish energy balance. The use of mussels as a satisfying mitigation tool reducing phytoplankton bloom impacts is discussed with regard to phytoplankton bloom magnitude and ashore marine fish farming in coastal ecosystems.

Community stoichiometry in a changing world: combined effects of warming and eutrophication on phytoplankton dynamics.

The current changes in our climate will likely have far-reaching consequences for aquatic ecosystems. These changes in the climate, however, do not act alone, and are often accompanied by additional stressors such as eutrophication. Both global warming and eutrophication have been shown to affect the timing and magnitude of phytoplankton blooms. Little is known about the combined effects of rising temperatures and eutrophication on the stoichiometry of entire phytoplankton communities. We exposed a natural phytoplankton spring community to different warming and phosphorus-loading scenarios using a full-factorial design. Our results demonstrate that rising temperatures promote the growth rate of an entire phytoplankton community. Furthermore, both rising temperatures and phosphorus loading stimulated the maximum biomass built up by the phytoplankton community. Rising temperatures led to higher carbon: nutrient stoichiometry of the phytoplankton community under phosphorus-limited conditions. Such a shift towards higher carbon: nutrient ratios, in combination with a higher biomass buildup, suggests a temperature-driven increase in nutrient use efficiency, the phytoplankton community. Importantly, with higher carbon: nutrient stoichiometry, phytoplankton is generally of poorer nutritional value for zooplankton. Thus, although warming may result in higher phytoplankton biomass, this may be accompanied by a stoichiometric mismatch between phytoplankton and their grazers, with possible consequences for the entire aquatic food web.

Spring and fall phytoplankton blooms in a productive subarctic ecosystem, the eastern Bering Sea, during 1995–2011

The timing and magnitude of phytoplankton blooms in subarctic ecosystems often strongly influence the amount of energy that is transferred through subsequent trophic pathways. In the eastern Bering Sea, spring bloom timing has been linked to ice retreat timing and production of zooplankton and fish. A large part of the eastern Bering Sea shelf (~500km wide) is ice-covered during winter and spring. Four oceanographic moorings have been deployed along the 70-m depth contour of the eastern Bering Sea shelf with the southern location occupied annually since 1995, the two northern locations since 2004 and the remaining location since 2001. Chlorophyll a fluorescence data from the four moorings provide 37 realizations of a spring bloom and 33 realizations of a fall bloom. We found that in the eastern Bering Sea: if ice was present after mid-March, spring bloom timing was related to ice retreat timing (p<0.001, df=1, 24); if ice was absent or retreated before mid-March, a spring bloom usually occurred in May or early June (average day 148, SE=3.5, n=11). A fall bloom also commonly occurred, usually in late September (average day 274, SE=4.2, n=33), and its timing was not significantly related to the timing of storms (p=0.88, df=1, 27) or fall water column overturn (p=0.49, df=1, 27). The magnitudes of the spring and fall blooms were correlated (p=0.011, df=28). The interval between the spring and fall blooms varied between four to six months depending on year and location. We present a hypothesis to explain how the large crustacean zooplankton taxa Calanus spp. likely respond to variation in the interval between blooms (spring to fall and fall to spring).

Wintertime controls on summer stratification and productivity at the western Antarctic Peninsula

We report results collected year‐round since 1998 in northern Marguerite Bay, just inside the Antarctic Circle. The magnitude of the spring phytoplankton bloom is much reduced following winters with reduced sea‐ice cover. In years with little winter sea‐ice the exposed sea surface leads to deep mixed layers in winter, and reduced water‐column stratification the following spring. Summer mixed‐layer depths are similar, however, so the change is not in overall light availability but toward a less stable water column with greater vertical mixing and increased variability in the light conditions experienced by phytoplankton. Macronutrient concentrations are replete at all times, but the increased vertical mixing likely reduces iron availability. The timing of bloom initiation is similar between heavy and light ice years, occurring soon after light returns in early spring, at a mixed‐layer averaged light level of &lt; 1 mol photon m−2 d−1. Ongoing regional climate change in the WAP area, and notably the ongoing loss of winter sea‐ice, is likely to drive a downward trend in the magnitude of phytoplankton blooms in this region of the Antarctic Peninsula.