Intense Blooms Research Articles

Increased temperature benefits growth and photosynthetic performance of the sea ice diatom Nitzschia cf. neglecta (Bacillariophyceae) isolated from saroma lagoon, Hokkaido, Japan.

During ice melt in spring, ice algae are released from the ice and could be exposed to variable temperatures and irradiances in surface water. Saroma Lagoon is an embayment with two inlets leading to the Sea of Okhotsk. With seasonal development of sea ice, its water temperature changes dramatically throughout the year. To investigate the living and photoprotective strategies of ice algae in such a coastal water system, we grew Nitzschia cf. neglecta, an ice diatom isolated from the sea ice of this lagoon, under irradiance levels of 30 and 100μmol photons·m-2 ·s-1 , and temperatures of 2°C and 10°C. Then the acclimated cells were exposed to high light in order to investigate the plasticity of their photosynthetic apparatus. At 10°C, cells grew faster and showed decreased susceptibility to high light. At 2°C, an immediate decrease in all pigment content upon exposure, as well as a higher cellular content of diatoxanthin was used to compensate for the more severe excitation stress. Highly efficient photoprotection was achieved through the diadinoxanthin-diatoxanthin cycle-dependent nonphotochemical quenching. While regulation through psbA and rbcL at the transcription level played a minor role in the response to high light stress at both temperatures. The wide tolerance to both temperature and light changes suggest that the thinning of sea ice and higher temperatures in a warmer world will lead to more intense blooms in Saroma Lagoon.

Extending the forecast model: Predicting Western Lake Erie harmful algal blooms at multiple spatial scales

Lake Erie is a classic case of development, recovery from, and return to eutrophication, hypoxia, and harmful algal blooms. Forecast models are used annually to predict bloom intensity for the whole Western Lake Erie Basin, but do not necessarily reflect nearshore conditions or regional variations, which are important for local stakeholders. In this study we: 1) developed relationships between observed whole basin and nearshore bloom sizes, and 2) updated and extended a Bayesian seasonal bloom forecast model to provide new regional predictions. The western basin was subdivided into 5 km near-shore regions, and bloom start date, size, and intensity were quantified with MODIS-derived images of chlorophyll concentrations for July–October 2002–2016 for each subdivision and for the entire basin. While bloom severity within each subdivision is temporally and spatially unique, it increased over the study period in each subdivision. The models for the 5 km subdivisions explained between 83 and 95% of variability between regional sizes and whole bloom size for US subdivisions and 51% for the Canadian subdivision. By linking predictive basin-wide models to regional regression estimates, we are now able to better predict potential bloom impacts at scales and in specific areas that are vital to the economic well-being of the region and allow for better management responses.

Anomalous summer-autumn phytoplankton bloom in 2015 in the Black Sea caused by several strong wind events

Abstract Four consecutive strong wind events in August 2015 caused an anomalous phytoplankton bloom in the Black Sea, which occupied almost the entire eastern basin. Maximal surface chlorophyll A concentration (Chl-A) in the bloom area exceeded 5 mg/m3 in comparison with 0.6 mg/m3 in the surrounding waters. The anomalous bloom was observed for over 3 months, from August till November. In this study, the physical mechanisms producing this long-lasting bloom are investigated using, satellite-derived optical, infrared, and altimetry measurements and in situ Bio-Argo data. The bloom resulted from the interaction of four physical factors: wind-induced turbulent entrainment; diapycnal mixing caused probably by the vertical shear of inertial currents; local Ekman upwelling at the left wind jet periphery; Ekman pumping of the basinscale cyclonic circulation. Wind induced vertical mixing deepened the mixed layer and entrained nutrients and Chl-A from the subsurface layers. At the same time, the maximum response of Chl-A occurred in a thin layer (~5 m) near the lower boundary of the subsurface chlorophyll peak. This feature results from diapycnal mixing, manifested in the weakening of the stratification in the upper 100 m, which caused the injection of nutrients into the euphotic layer. The mountains surrounding the enclosed Black Sea have an impact on the wind spatial structure generating the zones of wind jets and wind shadows. The wind gradients between areas of strong and weak winds produce local Ekman upwelling along the left periphery of the wind jet, with cyclonic shear, which strongly modify bloom spatial structure. At larger spatial scales, Ekman pumping alters the dynamical structure, producing a transition from “mesoscale” to “basin-scale” circulation regime. As a result, the Rim Current accelerates, uplifting the pycno-halocline and chemocline in the central basin. In the areas of upwelling, the elevated chemocline is eroded more intensely and turbulent entrainment provides nutrients and Chl-A to the surface. The most intense bloom occurred in the central region of the east cyclonic gyre and in mesoscale cyclones; blooms were absent in mesoscale anticyclones. Orbital velocities in both anticyclones and cyclones promoted the horizontal advection of nutrients and Chl-A on more than 200 km to the western part of the basin from the bloom centre.

Modulating the Effect of Iron and Total Organic Carbon on the Efficiency of a Hydrogen Peroxide-Based Algaecide for Suppressing Cyanobacteria

The intensity and frequency of cyanobacteria-dominated harmful algal blooms (cHABs) has been increasing. A key issue associated with cHABs is the potential to release cyanotoxins, such as microcystin. One of the primary methods for addressing cHABs in a reservoir is the application of algaecides. This research evaluated the impact of common environmental factors (i.e., Fe, total organic carbon) on the efficacy of a hydrogen peroxide-based algaecide to attain control of a targeted cyanobacterial population. The results found that sodium carbonate peroxydrate (SCP, trade name PAK®27) at half the manufacturer’s suggested application was effective at suppressing cyanobacteria for 2 weeks. For example, reactors that contained a full level of TOC and 1 mg/L Fe significantly decreased by 89% from 21,899 to 2437 ± 987 cells/mL (p < 0.05) by 2 days after treatment with half-dose SCP while reactors that contained the full-dose TOC and no SCP treatment depicted an increase in cyanobacteria population over the first week. Furthermore, as the cyanobacteria population decreased, the algal assemblage began to switch to being green algae dominant. Under the experimental conditions evaluated, Fe and total organic content did not interfere with the efficacy of SCP. SCP can provide effective control of cyanobacteria in a variety of environmental conditions.

Accumulation and biotransformation dynamics of the neurotoxic complex, saxitoxin, in different life stages of Ostrea chilensis

The neurotoxic complex saxitoxin, is a group of marine toxins that historically has significantly impacted human health and the ability to utilize marine resources. A steady increase in the distribution and intensity of Alexandrium catenella blooms in Chile, and around the world, has caused major ecological and socioeconomic impacts, putting this type of dinoflagellate, and its toxicity, in the spotlight. Ostrea chilensis is a commercially and ecologically important resource harvested from wild populations and farmed in centers of southern Chile, where it is exposed to large harmful algal blooms of the type that can cause paralysis in humans. This study contributes to our understanding about the transfer of toxins from A. catenella cells to juvenile and adult Ostrea chilensis by tracking transformations of the neurotoxic complex until it reaches its most stable molecular form in the intracellular environment of O. chilensis tissues. These biotransformations are different in O. chilensis juveniles and adults, indicating a differentiated response for these two life stages of this bivalve species. These studies can be used for similar analyses in other ecologically and commercially important species of filter feeding organisms, providing greater understanding of the specific interactions of bivalves in scenarios of toxic dinoflagellate proliferations (e.g. A. catenella blooms).

The Individual and Combined Effects of the Cyanotoxins, Anatoxin-a and Microcystin-LR, on the Growth, Toxin Production, and Nitrogen Fixation of Prokaryotic and Eukaryotic Algae.

Globally, eutrophication and warming of aquatic ecosystems has increased the frequency and intensity of cyanobacterial blooms and their associated toxins, with the simultaneous detection of multiple cyanotoxins often occurring. Despite the co-occurrence of cyanotoxins such as microcystins and anatoxin-a (ATX) in water bodies, their effects on phytoplankton communities are poorly understood. The individual and combined effects of microcystin-LR (MC-LR) and ATX on the cyanobacteria Microcystis spp., and Anabaena variabilis (a.k.a. Trichormus variabilis), and the chlorophyte, Selenastrum capricornutum were investigated in the present study. Cell density, chlorophyll-a content, and the maximum quantum efficiency of photosystem II (Fv/Fm) of Microcystis cells were generally lowered after exposure to ATX or MC-LR, while the combined treatment with MC-LR and ATX synergistically reduced the chlorophyll-a concentration of Microcystis strain LE-3. Intracellular levels of microcystin in Microcystis LE-3 significantly increased following exposure to MC-LR + ATX. The maximum quantum efficiency of photosystem II of Anabaena strain UTEX B377 declined during exposure to the cyanotoxins. Nitrogen fixation by Anabaena UTEX B377 was significantly inhibited by exposure to ATX, but was unaffected by MC-LR. In contrast, the combination of both cyanotoxins (MC-LR + ATX) caused a synergistic increase in the growth of S. capricornutum. While the toxins caused an increase in the activity of enzymes that scavenge reactive oxygen species in cyanobacteria, enzyme activity was unchanged or decreased in S. capricornutum. Collectively this study demonstrates that MC-LR and ATX can selectively promote and inhibit the growth and performance of green algae and cyanobacteria, respectively, and that the combined effect of these cyanotoxins was often more intense than their individual effects on some strains. This suggests that the release of multiple cyanotoxins in aquatic ecosystems, following the collapse of blooms, may influence the succession of plankton communities.

The role of water column stability and wind mixing in the production/export dynamics of two bays in the Western Antarctic Peninsula

During January and February 2017 massive phytoplankton blooms (chlorophyll > 15 mg m−3) were registered in surface waters within two bays in the Western Antarctic Peninsula (WAP). Reflecting these intense blooms, surface waters exhibited high pH (up to 8.4), low pCO2 (< 175 µatm) and low nitrate concentrations (down to 1.5 µM). These summer phytoplankton blooms consisted mainly of diatoms and were associated with the presence of shallow, surface freshwater plumes originating from glacier-melt outflow which contributed both to stratification and to iron supply, thus facilitating pronounced nitrate and CO2 drawdown. These findings suggest that with future increases in freshwater discharge around the WAP, phytoplankton blooms in the northern WAP may become more dominated by large cells, resembling the blooms occurring further south along the Peninsula. Fresher surface waters enhanced water column stability in both bays, enabling phytoplankton populations to attain high growth rates. Phytoplankton was observed to double their biomass in 2.3 days, consistent with the high net primary production rates recorded in both bays (1.29–8.83 g C m−2 d−1). Phytoplankton growth rates showed a direct mechanistic relationship with changes in water column stability, suggesting that this is a main driver of primary productivity in near-shore Antarctic coastal ecosystems, which agrees with previous findings. After wind induced mixing, the organic matter produced within both bays did not settle inside them, suggesting that it was laterally advected out of the bays. Thus, we hypothesize that highly productive near-shore bay areas in Antarctica may supply organic matter to oceanic waters.

Hydrophysical factors of coccolithophores Emiliania huxleyi intensive bloom formation in the Abkhazian sector of the Black Sea at the end of May 2013

At the end of May 2013, in the Abkhazian sector of the north-eastern part of the Black Sea, intensive blooms of Emiliania huxleyi coccolithophores of up to 4,4×106 cells/L were observed. In order to identify the necessary and sufficient hydrophysical conditions for the blooming of this alga at 15 stations of the water area, the spatial distribution of hydrophysical parameters was investigated. It is shown that the vertical distribution of temperature, salinity and density is typical for this time of year and is characterized by the presence of a sharp-gradient seasonal thermocline. The horizontal distribution of the main hydrophysical parameters on the surface water was as follows: at an average temperature of 20,209±1,027°C, the areas of elevated temperatures were located in the coastal zone, for salinity (17,585±0,556 psu) the reverse distribution was observed. Variations of conditional density were 11,477±0,516. A characteristic feature of 2013 was the renewal of the Cold Intermediate Layer (CIL) in the winter period. During the bloom period, the coccolithophores tended to be deepened along the coordinate from the center of the sea to the coast. Variations in the speed of geostrophic currents from 3,58 to 22,43 cm/s did not lead to the destruction of the bloom.

Surface Waters "Blooms" - Fundamental and Applied Aspects

The paper deals with the issues of the water "blooms" owing to mass development of Cyanoprokaryota, particularly in Ukraine. The reasons, dynamics, consequences of water "blooms", and factors influencing their development are discussed and analyzed. It is supposed that intensification of anthropogenic load and climate warming will result in the increase of intensity and frequency of the water blooms. Some biotechnological and physico-chemical approaches to the inhibition of the water "blooms" and its adverse effect, and also to the improvement of the water bodies state, including biological rehabilitation, algological and toxicological monitoring, are discussed.

Hydrophysical and Hydrochemical Controls of Cyanobacterial Blooms in Coursey Pond, Delaware (USA).

Noxious cyanobacterial blooms are common in many ponds in the mid-Atlantic Coastal Plain. In Delaware, blooms normally occur between July and October, yet no in-depth analyses of the causes and predictors exist. A study using commercially available, high-frequency, continuous, and automated biogeochemical sensors at Coursey Pond, Delaware, a pond known for perennial summer blooms, was conducted to investigate how hydrophysical and hydrochemical conditions affect bloom dynamics. Cyanobacterial abundance (based on the in vivo phycocyanin fluorescence and phycocyanin/chlorophyll fluorescence ratios) increases during periods of high water temperatures (up to 32°C), low discharge through the pond (mean hydraulic residence time ≥5 d) with evaporative concentration of dissolved solids, and decreasing NO concentrations (reaching <0.1 mg L, the detection limit). These conditions lead to the uptake and depletion of bioavailable N in the pond surface waters and provide a competitive advantage for temperature-tolerant and N-fixing cyanobacteria. Irrigation water use within the watershed can exceed pond discharge during drier summer months, enhancing bloom-forming conditions. Bloom intensity varies due to storms but persists until mid-October to mid-November when temperatures cool, daylength decreases, and discharge and NO concentration recovers. Changes in these easily monitored physical and chemical parameters can serve to anticipate the onset, intensity, persistence, and the eventual dissipation of cyanobacterial blooms at Coursey Pond and similar ponds elsewhere. Therefore, the use of sensors and high-frequency data has the potential to assist in forecasting and management of blooms.

Association between trophic state, watershed use, and blooms of cyanobacteria in south-central Chile

An increase in the distribution and frequency of cyanobacterial blooms has been reported for many regions worldwide. Due to this fact, we studied the variables that influence the abundance of natural populations of planktonic cyanobacteria in temperate lakes of central and southern Chile. These lakes differed in trophic state and watershed use. Cyanobacteria dominated in meso- and eutrophic systems and their occurrence correlated to watershed use (tree plantations and urban). Ochrophyta and Bacillariophyta were dominant in oligotrophic lakes, where native forest dominated land usage. In these lakes, the maximum depth of the euphotic zone influenced the community structure and the genera of cyanobacteria. Dolichospermum was the most abundant, frequent, and widely distributed genus, found in oligotrophic and eutrophic lakes, forming blooms in eutrophic systems.The concentration of total phosphorus and total nitrogen positively influenced cyanobacterial abundance and bloom formation, mainly by Aphanizomenon, Aphanocapsa, Aphanothece, and Dolichospermum, and Microcystis.In contrast to many reports on their occurrence in the northern hemisphere, these genera occurred widely at less than 20 °C, forming dispersive blooms, at low temperatures in autumn and winter (10.8–15.6 °C). This shows that eutrophication is the main factor for bloom formation and these genera can form blooms independent of temperature. However, some genera, such as Microcystis, increased their abundance and presented more intense blooms (scums) at high temperatures. Our study provides baseline data to document long-term changes in lentic systems of the western south-central area of South America, including genera that could respond by increasing their abundance with eutrophication and projected climatic changes.

Occurrence of winter phytoplankton bloom in the open southern Adriatic: Relationship with hydroclimatic events in the Eastern Mediterranean

The characteristics and intensity of winter phytoplankton blooms in the Open Southern Adriatic (OSA) were investigated from 1994 to 2012 by combining available unpublished and literature data on phytoplankton abundance, in situ chlorophyll-a (Chl-a) and satellite-derived Chl-a surface data. We used original data from Februaries 1994 and 1995 (phytoplankton, physico-chemical and meteorological data), the years affected by East Mediterranean Transient (EMT), and analysed them in relation to data available from literature for winters of 1997 and from 2007 to 2012. Chl-a satellite data from 1998 to 2012 was used to get insight into large scale bloom dynamics with fine temporal and spatial resolution. Particular attention was paid to the different circulation regimes in the Ionian Sea, namely to the anticyclonic and cyclonic phases of the Northern Ionian Gyre (NIG), both of which influence the physical and biochemical properties of the Southern Adriatic. The presented data demonstrate that winter blooms are persistent features of the OSA and they can occur during both anticyclonic and cyclonic phases of the Northern Ionian Gyre, but by different mechanisms. Moreover, intense blooms have occurred under certain hydroclimatic conditions such as the Eastern Mediterranean Transient (EMT) and extremely cold winter years. This was also confirmed by cluster analysis based on satellite Chl-a distribution.

Algal bloom transport in Lake Erie using remote sensing and hydrodynamic modelling: Sensitivity to buoyancy velocity and initial vertical distribution

In this study, we simulate three-dimensional transport of algal blooms in Lake Erie using a combination of remote sensing and hydrodynamic modelling. The remote sensing algorithms use data from the Sentinel-3 OLCI satellite sensor to derive chlorophyll-a concentration from cyanobacteria blooms in Lake Erie. The derived chlorophyll-a concentration initializes an algal bloom transport model driven by the lake component of the Water Cycle Prediction System for the Great Lakes, a system of coupled atmosphere-lake-hydrological models operated out of Environment and Climate Change Canada. The bloom is modelled as Microcystis aeruginosa, a buoyant species that is often dominant in harmful algal blooms in western Lake Erie. Short-term (a few days) predictions of algal bloom transport from July 27 to October 8, 2017 are modelled in both Eulerian and Lagrangian frameworks. The Eulerian framework is used to evaluate the sensitivity of model results to the initial vertical distribution of the bloom. In this work, the Lagrangian framework is limited to two-dimensional surface confined particles. We use several error metrics to evaluate model predictions. We find that results are sensitive to the buoyancy velocity for cases where the bloom was initially distributed over a large portion of the water column. An initial vertical distribution selected from modelled chlorophyll-a half depth shows the highest accuracy for the entire range of buoyancy velocities tested. We also find that the Pierce skill score is difficult to interpret, particularly in cases where bloom intensity is greatly overpredicted by the model.

Chemical oceanography of the Arabian Gulf

Most of the available information about basin-wide chemical processes in the Arabian Gulf is based on observations made during the expeditions of the Research Vessels Meteor (1965), Atlantis II (1977) and Umitaka-Maru (1993–1994) with the ongoing national programs in shallow territorial waters largely focusing on local environmental issues. Despite the acute paucity of data, it is clear that the Gulf's biogeochemistry and ecosystems are experiencing massive changes resulting from human activities. The two key components of these changes are a reduction in river runoff that has caused a large increase in salinity in the northern part of the Gulf, and an increase in nutrient loading largely through sewage discharge from the sprawling urban centers along the coasts. The change from a pristine, oligotrophic state to a human-perturbed, eutrophic system is not so clear from the long-term nutrient and chlorophyll trends, presumably because of rapid uptake of nutrients by the phytoplankton and consumption of the photosynthesized organic matter by the heterotrophs. However, eutrophication has led to large increases in the frequency and intensity of phytoplankton blooms since the 1990s. It may also account for the high total organic carbon (TOC) concentration in the water column and the recent development of summer-time hypoxia in near-bottom waters of the central Gulf, which have important implications for the future evolution of the oxygen minimum zone of the North-Western Indian Ocean. The high atmospheric dust deposition maintains high dissolved trace metal concentrations in seawater. There are several intriguing and hitherto unresolved aspects of nutrient budgets in the Arabian Gulf, mostly arising from the differences between nutrient concentrations in the inflowing and outflowing currents, highlighting the unknown roles and strengths of the local sources and sinks. However, even though the latter are being modified by human activities, the N:P concentrations in the deeper part of the Gulf vary in Redfieldian proportion. An interesting feature of nitrogen biogeochemistry in the Gulf is the accumulation of high nitrite in the euphotic zone. The relative contributions of nitrification and assimilatory nitrate reduction by phytoplankton to the nitrite build up are not known. Dissolved nitrous oxide concentrations are slightly above saturation. The extremely high thermohaline variability and biogenic carbonate production within the Gulf also make inorganic carbon cycling in the Gulf quite unusual. However, this remains one of the least studied topics of research in the Gulf. Another subject of disproportionately high significance is benthic biogeochemistry due to shallow depths and high temperatures within the Gulf. Since the small-sized Gulf is shared by eight countries, active regional cooperation is essential to understand its processes and come up with measures to address the serious threats faced by its biogeochemistry and ecosystems from human activities.

Harmful Algal Blooms Get “Hotter” in 2018

Harmful Algal Blooms Get “Hotter” in 2018

Responses of phytoplankton assemblages to iron availability and mixing water masses during the spring bloom in the Oyashio region, NW Pacific

AbstractSpring phytoplankton blooms play a major role in the carbon biogeochemical cycle of the Oyashio region, western subarctic Pacific, where the seasonal biological drawdown effect on seawater pCO2 is one of the greatest among the world's oceans. However, the bloom often terminates before depleting macronutrients, and the initiation and magnitude of the bloom is heterogeneous. We conducted a high resolution taxonomic and physiological assessment of phytoplankton in relation to the different physicochemical water masses of Coastal Oyashio Water (COW), Oyashio water (OYW), and modified Kuroshio water (MKW) in the Oyashio region from April to June 2007. Massive diatom blooms were found in April. Then, chlorophyll a concentration, cell abundance of diatom taxa, and the maximum photosystem II photochemical efficiency (Fv/Fm) were positively correlated with the mixing ratios of COW, suggesting that the spring bloom in April was strongly affected by the intrusion of COW. In the OYW, intensive blooms occurred from the middle of May under low dissolved iron (DFe) concentration (&lt; 0.26 nM). Redundancy analysis showed that while diatom blooms accompanied by COW were related to DFe concentration, this was not the case in the OYW. These results indicated that diatoms in the OYW possess different iron adaptation strategies compared with diatoms in the water masses affected by COW. This led to the spatial heterogeneity of the Oyashio spring bloom. The results presented here demonstrate that water mass characterization with detailed assessments of phytoplankton taxonomy and physiological status can improve our understanding of marine ecosystems.

Multispecies Fruit Flower Detection Using a Refined Semantic Segmentation Network

In fruit production, critical crop management decisions are guided by bloom intensity, i.e., the number of flowers present in an orchard. Despite its importance, bloom intensity is still typically estimated by means of human visual inspection. Existing automated computer vision systems for flower identification are based on hand-engineered techniques that work only under specific conditions and with limited performance. This work proposes an automated technique for flower identification that is robust to uncontrolled environments and applicable to different flower species. Our method relies on an end-to-end residual convolutional neural network (CNN) that represents the state-of-the-art in semantic segmentation. To enhance its sensitivity to flowers, we fine-tune this network using a single dataset of apple flower images. Since CNNs tend to produce coarse segmentations, we employ a refinement method to better distinguish between individual flower instances. Without any pre-processing or dataset-specific training, experimental results on images of apple, peach and pear flowers, acquired under different conditions demonstrate the robustness and broad applicability of our method.

Transgenerational effects of cyanobacterial toxins on a tropical micro-crustacean Daphnia lumholtzi across three generations

Climate change and human activities induce an increased frequency and intensity of cyanobacterial blooms which could release toxins to aquatic ecosystems. Zooplankton communities belong to the first affected organisms, but in tropical freshwater ecosystems, this issue has yet been poorly investigated. We tested two questions (i) if the tropical Daphnia lumholtzi is capable to develop tolerance to an ecologically relevant concentration of purified microcystin-LR and microcystins from cyanobacterial extract transferable to F1 and F2 generations? And (ii) would F1 and F2 generations recover if reared in toxin-free medium? To answer these questions, we conducted two full factorial mutigenerational experiments, in which D. lumholtzi was exposed to MC-LR and cyanobacterial extract at the concentration of 1 μg L−1 microcystin continuously for three generations. After each generation, each treatment was spit into two: one reared in the control (toxin free) while the other continued in the respective exposure. Fitness-related traits including survival, maturity age, body length, and fecundity of each D. lumholtzi generation were quantified. Though there were only some weak negative effects of the toxins on the first generation (F0), we found strong direct, accumulated and carried-over impacts of the toxins on life history traits of D. lumholtzi on the F1 and F2, including reductions of survival, and reproduction. The maturity age and body length showed some inconsistent patterns between generations and need further investigations. The survival, maturity age (for extract), and body length (for MC-LR) were only recovered when offspring from toxin exposed mothers were raised in clean medium for two generations. Chronic exposure to long lasting blooms, even at low density, evidently reduces survival of D. lumholtzi in tropical lakes and reservoirs with ecological consequences.

In situ ingestion of Microcystis is negatively related to copepod abundance in the upper San Francisco Estuary

AbstractAlthough zooplankton grazing on cyanobacteria is a key process regulating the structure and function of bloom‐dominated waters, few studies have measured in situ ingestion and none have linked it to zooplankton abundance. This study measured in situ zooplankton grazing rate on Microcystis via quantitative polymerase chain reaction (qPCR) amplification of prey DNA from the guts of the copepod Pseudodiaptomus forbesi, an abundant and important prey species for endangered fish in the upper San Francisco Estuary (SFE). We calibrated the qPCR assay in laboratory grazing experiments with Eurytemora affinis, a similar sized copepod abundant in the SFE during winter, due to higher ingestion of Microcystis by this copepod compared to Pseudodiaptomus forbesi. In the field, Pseudodiaptomus forbesi ingested Microcystis during blooms, including strains capable of producing microcystin toxins. Although the Microcystis content in Pseudodiaptomus forbesi guts increased with bloom intensity, estimated ingestion rates were low and suggested copepods avoided this cyanobacterium. Moreover, Pseudodiaptomus forbesi abundance was inversely related to its gut Microcystis content, but not related to other factors including temperature, salinity, turbidity, and gut fluorescence of chlorophyll a. Results demonstrate that the gut Microcystis content of zooplankton may be a key factor explaining their abundance in nature. Our results also highlight the utility of molecular methods for understanding in situ trophic interactions in bloom‐dominated waters. Increased blooms under future climate scenarios (e.g., extreme drought and higher temperatures) may decrease the abundance of key zooplankton like Pseudodiaptomus forbesi through increased ingestion of Microcystis, with potentially negative effects on endangered fishes in the SFE.

Microzooplankton in the coastal Gulf of Alaska: Regional, seasonal and interannual variations

Microzooplankton communities in the coastal Gulf of Alaska (CGOA) were studied during two contrasting years: 2011, with a greatly reduced spring phytoplankton bloom, and 2013, with a robust spring bloom. Other sampling contrasts were season (spring, summer, fall) and region (eastern versus western shelf waters). Ciliates and dinoflagellates comprised nearly all microzooplankton in the ≥ 15 µm size class. Many of the strongest contrasts in the biomass and taxonomic composition of the microzooplankton community were regional. The east had generally lower microzooplankton biomass levels and a greater proportion of ciliates than the west, even in the face of basin-wide seasonal and interannual contrasts. This difference is likely a consequence of the narrower shelf in the east, which leads to a lower productivity environment. Interannual differences in spring bloom intensity were reflected in microzooplankton biomass (higher in spring 2013, especially in the east), while interannual differences in taxonomic composition persisted throughout the year, with a greater representation of ciliates in 2011. Ciliate dominance could reflect adaptations to lower productivity conditions, with many of the largest taxa likely retaining chloroplasts as a strategy (mixotrophy) for survival during times of prey scarcity. Microzooplankton: phytoplankton biomass ratios varied widely over time and space. These ratios indicate that lower productivity regions (east) and seasons (summer) also tend to be locations and times of reduced trophic transfer efficiency from phytoplankton to ciliates and dinoflagellates. Ciliates and dinoflagellates can be preferred prey of mesozooplankton, including dominant CGOA copepod species. Thus multiple mechanisms conspire to reduce the flow of matter and energy to higher trophic levels in low productivity locations and time periods in the CGOA.