Shifts In Phytoplankton Community Structure Research Articles

A lag bloom pattern of phytoplankton after freshwater input events revealed by daily samples during summer in Qinhuangdao coastal water, China.

Phytoplankton blooms have become a global concern due to their negative impacts on public health, aquaculture, tourism, and the economic stability of coastal regions. Therefore, elucidating the shifts in phytoplankton community structure and abundance, as well as their environmental drivers, is crucial. However, existing studies often fail to capture the detailed dynamics of phytoplankton blooms and their environmental triggers due to low temporal observation resolution. In this study, high temporal resolution (daily) samples were collected over 43 days to investigate the influence of environmental factors on phytoplankton in Qinhuangdao in the summer. During the observation period, a total of 45 phytoplankton species were identified, comprising 26 Bacillariophyta species, 16 Dinophyta species, 2 Euglenophyta species, and 1 Chromophyta species. Interestingly, a lag bloom pattern of phytoplankton behind freshwater input was observed across day-to-day samples. Phytoplankton blooms typically lagged 1-3 days behind periods of decreased salinity and nutrient input, suggesting that freshwater influx provides the foundational materials and benefits for these blooms. Moreover, the phytoplankton blooms were triggered by six dominant species, i.e., Chaetoceros spp., Pseudo-nitzschia delicatissima, Skeletonema costatum, Protoperdinium spp., Leptocylindrus minimus, Pseudo-nitzschia pungens, and Thalassiosira spp. Consequently, the succession of phytoplankton showed a predominant genera shift in the following sequence: Nitzschia, Protoperdinium, and Prorocentrum - Skeletonema - Pseudo-nitzschia - Gymnodinium - Leptocylindrus. Besides that, a deterministic process dominated phytoplankton community assembly across time series, and DIP is a key factor in shifting the phytoplankton community structures in this area. In summary, our study offers high-resolution observations on the succession of phytoplankton communities and sheds light on the complex and differentiated responses of phytoplankton to environmental factors. These findings enhance our understanding of the dynamics of phytoplankton blooms and their environmental drivers, which is essential for the effective management and mitigation of their adverse impacts.

Seasonal cycles of biogeochemical fluxes in the Scotia Sea, Southern Ocean: a stable isotope approach

Abstract. The biological carbon pump is responsible for much of the decadal variability in the ocean carbon dioxide (CO2) sink, driving the transfer of carbon from the atmosphere to the deep ocean. A mechanistic understanding of the ecological drivers of particulate organic carbon (POC) flux is key both to the assessment of the magnitude of the ocean CO2 sink and for accurate predictions as to how this will change with changing climate. This is particularly important in the Southern Ocean, a key region for the uptake of CO2 and the supply of nutrients to the global thermocline. In this study we examine sediment-trap-derived particle fluxes and stable isotope signatures of carbon (C), nitrogen (N), and biogenic silica (BSi) at a study site in the biologically productive waters of the northern Scotia Sea in the Southern Ocean. Both deep (2000 m) and shallow (400 m) sediment traps exhibited two main peaks in POC, particulate N, and BSi flux: one in austral spring and one in summer, reflecting periods of high surface productivity. Particulate fluxes and isotopic compositions were similar in both deep and shallow sediment traps, highlighting that most remineralisation occurred in the upper 400 m of the water column. Differences in the seasonal cycles of isotopic compositions of C, N, and Si provide insights into the degree of coupling of these key nutrients. We measured increasing isotopic enrichment of POC and BSi in spring, consistent with fractionation during biological uptake. Since we observed isotopically light particulate material in the traps in summer, we suggest physically mediated replenishment of lighter isotopes of key nutrients from depth, enabling the full expression of the isotopic fractionation associated with biological uptake. The change in the nutrient and remineralisation regimes, indicated by the different isotopic compositions of the spring and summer productive periods, suggests a change in the source region of material reaching the traps and associated shifts in phytoplankton community structure. This, combined with the occurrence of advective inputs at certain times of the year, highlights the need to make synchronous measurements of physical processes to improve our ability to track changes in the source regions of sinking particulate material. We also highlight the need to conduct particle-specific (e.g. faecal pellets, phytoplankton detritus, zooplankton moults) isotopic analysis to improve the use of this tool in assessing particle composition of the sinking material and to develop our understanding of the drivers of biogeochemical fluxes.

Climate drivers of Southern Ocean phytoplankton community composition and potential impacts on higher trophic levels

Southern Ocean phytoplankton production supports rich Antarctic marine ecosystems comprising copepods, krill, fish, seals, penguins, and whales. Anthropogenic climate change, however, is likely to drive rearrangements in phytoplankton community composition with potential ramifications for the whole ecosystem. In general, phytoplankton communities dominated by large phytoplankton, i.e., diatoms, yield shorter, more efficient food chains than ecosystems supported by small phytoplankton. Guided by a large ensemble of Earth system model simulations run under a high emission scenario (RCP8.5), we present hypotheses for how anthropogenic climate change may drive shifts in phytoplankton community structure in two regions of the Southern Ocean: the Antarctic Circumpolar Current (ACC) region and the sea ice zone (SIZ). Though both Southern Ocean regions experience warmer ocean temperatures and increased advective iron flux under 21st century climate warming, the model simulates a proliferation of diatoms at the expense of small phytoplankton in the ACC, while the opposite patterns are evident in the SIZ. The primary drivers of simulated diatom increases in the ACC region include warming, increased iron supply, and reduced light from increased cloudiness. In contrast, simulated reductions in ice cover yield greater light penetration in the SIZ, generating a phenological advance in the bloom accompanied by a shift to more small phytoplankton that effectively consume available iron; the result is an overall increase in net primary production, but a decreasing proportion of diatoms. Changes of this nature may promote more efficient trophic energy transferviacopepods or krill in the ACC region, while ecosystem transfer efficiency in the SIZ may decline as small phytoplankton grow in dominance, possibly impacting marine food webs sustaining Antarctic marine predators. Despite the simplistic ecosystem representation in our model, our results point to a potential shift in the relative success of contrasting phytoplankton ecological strategies in different regions of the Southern Ocean, with ramifications for higher trophic levels.

A New Ocean State After Nuclear War

AbstractNuclear war would produce dire global consequences for humans and our environment. We simulated climate impacts of US‐Russia and India‐Pakistan nuclear wars in an Earth System Model, here, we report on the ocean impacts. Like volcanic eruptions and large forest fires, firestorms from nuclear war would transport light‐blocking aerosols to the stratosphere, resulting in global cooling. The ocean responds over two timescales: a rapid cooling event and a long recovery, indicating a hysteresis response of the ocean to global cooling. Surface cooling drives sea ice expansion, enhanced meridional overturning, and intensified ocean vertical mixing that is expanded, deeper, and longer lasting. Phytoplankton production and community structure are highly modified by perturbations to light, temperature, and nutrients, resulting in initial decimation of production, especially at high latitudes. A new physical and biogeochemical ocean state results, characterized by shallower pycnoclines, thermoclines, and nutriclines, ventilated deep water masses, and thicker Arctic sea ice. Persistent changes in nutrient limitation drive a shift in phytoplankton community structure, resulting in increased diatom populations, which in turn increase iron scavenging and iron limitation, especially at high latitudes. In the largest US‐Russia scenario (150 Tg), ocean recovery is likely on the order of decades at the surface and hundreds of years at depth, while changes to Arctic sea‐ice will likely last thousands of years, effectively a “Nuclear Little Ice Age.” Marine ecosystems would be highly disrupted by both the initial perturbation and in the new ocean state, resulting in long‐term, global impacts to ecosystem services such as fisheries.

CORRIGENDUM: Shifts in phytoplankton community structure modify bacterial production, abundance and community composition

CORRIGENDUM: Shifts in phytoplankton community structure modify bacterial production, abundance and community composition

Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean

Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean

Diatom and coccolithophore species fluxes in the Subtropical Frontal Zone, east of New Zealand

The Chatham Rise supports some of New Zealand's most economically valuable commercial fish stocks, fuelled by the highly productive waters of the Subtropical Frontal Zone (STFZ). Climate change-related shifts in phytoplankton community structure and function are predicted and may affect Chatham Rise productivity and deep-sea carbon sequestration by the biological carbon pump. However, little is known about how two major phytoplankton groups, diatoms and coccolithophores, will respond, since the seasonality of their productivity and export fluxes to the seafloor are not well known in the SW Pacific sector. This study is the first to report on the seasonal cycle of phytoplankton species fluxes in the STFZ, using a 12-month sediment trap record (1996–1997) on the northern (subtropical-influenced) and southern (subantarctic-influenced) flanks of the Chatham Rise. Diatom and coccolith flux assemblages were characterised and compared with biogeochemical fluxes from the same sites over the same time-frame. Northern phytoplankton flux composition was typical of the subtropics, with most particulate organic carbon (POC) and biogenic silica export associated with diatom fluxes. Fluxes to the northern trap were largely composed of coastal and benthic diatom taxa, reinforcing previous interpretations suggesting lateral advection and sediment resuspension along and across the Chatham Rise slope, influenced also by the Wairarapa Eddy. At the southern site, phytoplankton fluxes were on average an order of magnitude higher than at the northern site and comprised a combination of subantarctic and frontal zone diatom taxa. The majority of the annual diatom flux in the southern trap occurred during a spring Pseudo-nitzschia “pulse” associated with high biogenic silica and POC fluxes, which are inferred to be from subsurface chlorophyll maxima. High coccolith flux at the southern STFZ site was consistent with ecosystem processes observed from other subantarctic export regimes. Phytoplankton successions inferred from the sediment trap records suggest that coccolithophores precede diatoms in spring, which is contrary to expectations from classical studies where the opposite temporal trend is suggested. While maximum diatom species flux co-occurred with peak POC flux at both sites in spring, the estimated contribution of organic carbon associated with diatoms to POC flux was negligible (<0.05%). Estimated coccolithophore POC flux was also low, suggesting that either other phytoplankton groups and/or zooplankton-mediated processes (e.g. fecal pellet fluxes) must contribute more substantially to POC export in the STFZ. Our findings provide new insights into phytoplankton export flux pathways to the seafloor in the Chatham Rise region, with implications for regional biogeochemistry, paleoceanographic and export modelling studies.

Shifts in Phytoplankton Community Structure Across an Anticyclonic Eddy Revealed From High Spectral Resolution Lidar Scattering Measurements

Changes in airborne high spectral resolution lidar (HSRL) measurements of scattering, depolarization, and attenuation coincided with a shift in phytoplankton community composition across an anticyclonic eddy in the North Atlantic. We normalized the total depolarization ratio (delta) by the particulate backscattering coefficient (b(b)(p)) to account for the covariance in delta and b(b)(p) that has been attributed to multiple scattering. A 15% increase in delta/b(b)(p) inside the eddy coincided with decreased phytoplankton biomass and a shift to smaller and more elongated phytoplankton cells. Taxonomic changes (reduced dinoflagellate relative abundance inside the eddy) were also observed. The delta signal is thus potentially most sensitive to changes in phytoplankton shape because neither the observed change in the particle size distribution (PSD) nor refractive index (assuming average refractive indices) are consistent with previous theoretical modeling results. We additionally calculated chlorophyll-a (Chl) concentrations from measurements of the diffuse light attenuation coefficient (K-d) and divided by b(b)(p) to evaluate another optical metric of phytoplankton community composition (Chl:b(bp)), which decreased by more than a factor of two inside the eddy. This case study demonstrates that the HSRL is able to detect changes in phytoplankton community composition. High spectral resolution lidar measurements reveal complex structures in both the vertical and horizontal distribution of phytoplankton in the mixed layer providing a valuable new tool to support other remote sensing techniques for studying mixed layer dynamics. Our results identify fronts at the periphery of mesoscale eddies as locations of abrupt changes in near-surface optical properties.

Evaluation of Water Management Effectiveness by a Multi-Dimensional Approach

As water is utilized in various ways across the whole spectrum of human life, better understanding of water management effectiveness and sharing of information and experience are crucial for achieving sustainability from a water perspective. However, water management effectiveness in the long term has been insufficiently studied due partially to the lack of long term data and methodology for long term assessment. The present study presents a case study, dealing with the environmental restoration of a once listed as the worst lake in Japan for 27 years through a water diversion project. It highlights the need and importance of evaluating water management practice from both chemical and biological dimensions for its long-term effectiveness. By combining field surveys of phytoplankton communities and E. coli at multiple locations in the lake in different seasons and data analysis of long term change of chemical parameters of water quality, it revealed that although the diversion project led to a shift in phytoplankton community structure from being bluegreen algae dominated to diatom dominated, the eutrophic level remains high and the diatom-dominated phytoplankton community structure is probably the factor hindering the true recovery of the ecosystem in the lake. Wording differently, the diversion operation alleviated the symptom of eutrophication but did not solve the problem. A message form the present study is that such a multi-dimensional assessment approach allows the explanation of water management effectiveness in ecological relevance and identification of the direction for further improvement work. Therefore, it also serves as a call for the further development of multi-dimensional assessment approaches.

Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities.

Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open-ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ13 C values of 0.8‰-2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel-derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in 13 C-rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna δ13 C values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota.

The Phytoplankton Taxon-Dependent Oil Response and Its Microbiome: Correlation but Not Causation.

Phytoplankton strongly interact with their associated bacteria, both attached (PA), and free-living (FL), and bacterial community structures can be specific to phytoplankton species. Similarly, responses to environmental stressors can vary by taxon, as exemplified by observed shifts in phytoplankton community structure from diatoms to phytoflagellates after the Deepwater Horizon (DWH) oil spill. Here, we assess the extent to which associated bacteria influence the phytoplankton taxon-specific oil response by exposing xenic and axenic strains of three phytoplankton species to oil and/or dispersant. The dinoflagellates Amphidinium carterae and Peridinium sociale, and the diatom Skeletonema sp., all harbored significantly distinct bacterial communities that reflected their host oil response. Oil degrading bacteria were detected in both PA and FL communities of the oil resistant dinoflagellates, but their FL bacteria were more efficient in lipid hydrolysis, a proxy for oil degradation capability. Inversely, the growth rate and photosynthetic parameters of the diatom Skeletonema sp. was the most impacted by dispersed oil compared to the dinoflagellates, and oil-degrading bacteria were not significantly associated to its microbiome, even in the dispersed oil treatment. Moreover, the FL bacteria of Skeletonema did not show significant oil degradation. Yet, the lack of consistent significant differences in growth or photosynthetic parameters between the xenic and axenic cultures after oil exposure suggest that, physiologically, the associated bacteria do not modify the phytoplankton oil response. Instead, both oil resistance and phycosphere composition appear to be species-specific characteristics that are not causally linked. This study explores one aspect of what is undoubtedly a complex suite of interactions between phytoplankton and their associated bacteria; future analyses would benefit from studies of genes and metabolites that mediate algal-bacterial exchanges.

Response of Subtropical Phytoplankton Communities to Ocean Acidification Under Oligotrophic Conditions and During Nutrient Fertilization

The subtropical oceans are home to the largest phytoplankton biome on the planet. Yet, little is known about potential impacts of ocean acidification (OA) on phytoplankton community composition in the vast oligotrophic ecosystems of the subtropical gyres. To address this question, we conducted an experiment with 9 in situ mesocosms (~35 m3) off the coast of Gran Canaria in the eastern subtropical North Atlantic over a period of 9 weeks. By establishing a gradient of pCO2 ranging from ~350 to 1025 µatm, we simulated carbonate chemistry conditions as projected until the end of the 21st century. Furthermore, we injected nutrient-rich deep water into the mesocosms halfway through the experiment to simulate a natural upwelling event, which regularly leads to patchy nutrient fertilization in the study region. The temporal developments of major taxonomic groups of phytoplankton were analyzed by flow cytometry, pigment composition and microscopy. We observed distinct shifts in phytoplankton community structure in response to high CO2, with markedly different patterns depending on nutrient status of the system. Phytoplankton biomass during the oligotrophic phase was dominated by picocyanobacteria (Synechococcus), which constituted 60-80% of biomass and displayed significantly higher cell abundances at elevated pCO2. The addition of deep water triggered a substantial bloom of large, chain-forming diatoms (mainly Guinardia striata and Leptocylindrus danicus) that dominated the phytoplankton community during the bloom phase (70-80% of biomass) and until the end of the experiment. A CO2 effect on bulk diatom biomass became apparent only in the highest CO2 treatments (>800 µatm), displaying elevated concentrations especially in the stationary phase after nutrient depletion. Notably, these responses were tightly linked to distinct interspecific shifts within the diatom assemblage, particularly favoring the largest species Guinardia striata. Other taxonomic groups contributed less to total phytoplankton biomass, but also displayed distinct responses to OA treatments. For instance, higher CO2 favored the occurrence of prymnesiophyceae (Phaeocystis globosa) and dictyochophyceae, whereas dinoflagellates were negatively affected by increasing CO2. Altogether, our findings revealed considerable shifts in species composition in response to elevated CO2 and indicated that phytoplankton communities in the subtropical oligotrophic oceans might be profoundly altered by ocean acidification.

Shifts in phytoplankton community structure in response to hydrological changes in the shallow St Lucia Estuary

Hydrological regimes are key in shaping the structure of phytoplankton communities in estuaries. Because anthropogenic disturbances often alter the quantity and quality of riverine inputs, this research focused on investigating phytoplankton dynamics in the St. Lucia Estuary, since being reconnected to the Mfolozi River. Due to the closed mouth state of the estuary, the input from the river resulted in oligohaline (≥5) conditions and the characteristic reverse salinity gradient to persist throughout the study (i.e. one year). Model results indicated that phytoplankton communities were structured by rainfall, light transmittance and depth, while shifts between dominant groups were driven by salinity and flow season. The persistence of chlorophytes and euglenophytes as the co-dominant functional group indicates the freshening and nutrient-enriched nature of the river. This study provides the first detailed account documenting the response of the phytoplankton community to recent restoration efforts in the St Lucia Estuary, a Ramsar Site.

Shifts in phytoplankton community structure modify bacterial production, abundance and community composition

Shifts in phytoplankton community structure modify bacterial production, abundance and community composition

Biological Nitrogen Fixation Prevents the Response of a Eutrophic Lake to Reduced Loading of Nitrogen: Evidence from a 46-Year Whole-Lake Experiment

A whole-ecosystem experiment in Lake 227 (L227) at the Experimental Lakes Area, ongoing since 1969, examined the roles of carbon (C), nitrogen (N), and phosphorus (P) in controlling eutrophication. During 2011, we conducted a series of sub-experiments and more intensive monitoring to improve estimates of N fixation and its ability to meet algal growth demands in the decades following the cessation of artificial N loading, while maintaining long-term high artificial P loading. Stoichiometric nutrient ratios indicated both moderate N and P limitation of the phytoplankton during spring, preceding a shift in phytoplankton community structure toward dominance by N fixing cyanobacteria. During bloom development, and for the remainder of the stratified period, stoichiometric nutrient ratios indicated moderate to strong P limitation. N fixation rates, corrected using 15N2 methods, increased 2× after 1990, when N loading ceased. Ambient dissolved inorganic nitrogen prior to the bloom represented less than 3% of N demands of the phytoplankton. N fixation accounted for between 69–86% of total N loading to the epilimnion during the period of rapid bloom development, and 72–86% of total N loading during the May–October period. Phytoplankton biomass did not decline in L227 during the 40 years since artificial N loading was reduced, or the nearly 25 years since artificial N loads ceased entirely (1990–2013), and remained approximately 20× higher than four nearby reference lakes. These results suggest that despite constraints on biological N fixation, it retains a large capacity to offset potential N loading reductions in freshwaters.

Climate Effects on High Latitude Daphnia via Food Quality and Thresholds.

Climate change is proceeding rapidly at high northern latitudes and may have a variety of direct and indirect effects on aquatic food webs. One predicted effect is the potential shift in phytoplankton community structure towards increased cyanobacterial abundance. Given that cyanobacteria are known to be a nutritionally poor food source, we hypothesized that such a shift would reduce the efficiency of feeding and growth of northern zooplankton. To test this hypothesis, we first isolated a clone of Daphnia pulex from a permafrost thaw pond in subarctic Québec, and confirmed that it was triploid but otherwise genetically similar to a diploid, reference clone of the same species isolated from a freshwater pond in southern Québec. We used a controlled flow-through system to investigate the direct effect of temperature and indirect effect of subarctic picocyanobacteria (Synechococcus) on threshold food concentrations and growth rate of the high latitude clone. We also compared the direct effect of temperature on both Daphnia clones feeding on eukaryotic picoplankton (Nannochloropsis). The high latitude clone had a significantly lower food threshold for growth than the temperate clone at both 18 and 26°C, implying adaptation to lower food availability even under warmer conditions. Polyunsaturated fatty acids were present in the picoeukaryote but not the cyanobacterium, confirming the large difference in food quality. The food threshold for growth of the high latitude Daphnia was 3.7 (18°C) to 4.2 (26°C) times higher when fed Synechococcus versus Nannochloropsis, and there was also a significant negative effect of increased temperature and cyanobacterial food on zooplankton fatty acid content and composition. The combined effect of temperature and food quality on the performance of the high latitude Daphnia was greater than their effects added separately, further indicating the potentially strong indirect effects of climate warming on aquatic food web processes.

Timing of sea ice retreat can alter phytoplankton community structure in the western Arctic Ocean

Abstract. This study assesses the response of phytoplankton assemblages to recent climate change, especially with regard to the shrinking of sea ice in the northern Chukchi Sea of the western Arctic Ocean. Distribution patterns of phytoplankton groups in the late summers of 2008–2010 were analysed based on HPLC pigment signatures and, the following four major algal groups were inferred via multiple regression and cluster analyses: prasinophytes, diatoms, haptophytes and dinoflagellates. A remarkable interannual difference in the distribution pattern of the groups was found in the northern basin area. Haptophytes dominated and dispersed widely in warm surface waters in 2008, whereas prasinophytes dominated in cold water in 2009 and 2010. A difference in the onset date of sea ice retreat was evident among years–the sea ice retreat in 2008 was 1–2 months earlier than in 2009 and 2010. The spatial distribution of early sea ice retreat matched the areas in which a shift in algal community composition was observed. Steel-Dwass's multiple comparison tests were used to assess the physical, chemical and biological parameters of the four clusters. We found a statistically significant difference in temperature between the haptophyte-dominated cluster and the other clusters, suggesting that the change in the phytoplankton communities was related to the earlier sea ice retreat in 2008 and the corollary increase in sea surface temperatures. Longer periods of open water during the summer, which are expected in the future, may affect food webs and biogeochemical cycles in the western Arctic due to shifts in phytoplankton community structure.

Influence of the Pacific Decadal Oscillation on phytoplankton phenology and community structure in the western North Pacific

Phytoplankton phenology and community structure in the western North Pacific were investigated for 2001–2009, based on satellite ocean colour data and the Continuous Plankton Recorder survey. We estimated the timing of the spring bloom based on the cumulative sum satellite chlorophyll adata, and found that the Pacific Decadal Oscillation (PDO)‐related interannual SST anomaly in spring significantly affected phytoplankton phenology. The bloom occurred either later or earlier in years of positive or negative PDO (indicating cold and warm conditions, respectively). Phytoplankton composition in the early summer varied depending on the magnitude of seasonal SST increases, rather than the SST value itself. Interannual variations in diatom abundance and the relative abundance of non‐diatoms were positively correlated with SST increases for March–April and May–July, respectively, suggesting that mixed layer environmental factors, such as light availability and nutrient stoichiometry, determine shifts in phytoplankton community structure. Our study emphasised the importance of the interannual variation in climate‐induced warm–cool cycles as one of the key mechanisms linking climatic forcing and lower trophic level ecosystems.

Impact of the 2009 extreme water level variation on phytoplankton community structure in Lower Amazon floodplain lakes

AIM: This paper examines the effect of the extreme water level change in 2009 on the structure and diversity of the phytoplankton communities in lakes of the Lower Amazon Floodplain, and compares it to phytoplankton community structure data reported in the literature for 2002 and 2003 high water periods, closer to the normal hydrological conditions. METHODS: Sub-surface integrated water samples for phytoplankton and chlorophyll-a analyses were collected during high and low water phases in 2009. Water temperature (°C), pH, turbidity (NTU) and electrical conductivity (µS.m-1) were measured, and the Shannon diversity index was calculated. RESULTS: The results showed striking differences in taxonomic composition between phases (high and low) and also between normal (2002 and 2003) and extreme (2009) hydrological conditions, all related to the flood pulse intensity. CONCLUSIONS: Extreme water level fluctuations can result in shifts in phytoplankton community structure and diversity. This work represents a valuable contribution to phytoplankton research since presents the community structure under extreme hydrological events in the Amazon floodplain.

Shift in Phytoplankton Community Structure in a Tropical Marine Reserve Before and After a Major oil Spill Event

The present study reports the changes in the phytoplankton community structure in Taklong Island National Marine Reserve (TINMAR), Guimaras Island, Philippines. Quantification of PAH yielded undetectable results, whereas, primary productivity, phytoplankton density, and diversity values were higher as compared to samples before the oil spill and samples from the reference site. Sixty-nine genera representing 6 classes of phytoplankton were identified. Class distribution revealed that diatoms belonging to Coscinodiscophyceae, Bacillariophyceae and Fragilariophyceae were dominant in the area. Class Coscinodiscophyceae was the best represented class with 1,535 individuals/L seawater. The top-ranked diatom genera encountered were Chaetoceros, Skeletonema, Thalassionema, Rhizosolenia, and Bacteriastrum. The shifts in dominance of diatoms over dinoflagellates and fast-growing centric diatoms over pennate diatoms are indicative of a stressed phytoplankton community. Both the Simpsons (1/D’) and Shannon-Weiner (H’) values registered for the 2006 TINMAR samples (1/D’:11.23; H’:1.304) were higher than those obtained from pre-oil impacted samples (1/D’:8.83; H’:1.07) and samples from the reference site (1/D’:8.798; H’:1.039). The present findings provided information on the direct impact of a recent oil spill on phytoplankton community and demonstrate the suitability of using phytoplankton as bioindicators of environmental stress.