Increase In Phytoplankton Biomass Research Articles

Response of coastal phytoplankton to pollution from various sources in the coastal Bay of Bengal.

The coastal ocean receives nutrient pollutants from various sources, such as aerosols, municipal sewage, industrial effluents and groundwater discharge, with variable concentrations and stoichiometric ratios. The objective of this study is to examine the response of phytoplankton to these pollutants in the coastal water under silicate-rich and silicate-poor coastal waters. In order to achieve this, a microcosm experiment was conducted by adding the pollutants from various sources to the coastal waters during November and January, when the water column physicochemical characteristics are different. Low salinity and high silicate concentration were observed during November due to the influence of river discharge contrasting to that observed during January. Among the various sources of pollutants used, aerosols and industrial effluents did not contribute silicate whereas groundwater and municipal sewage contained high concentrations of silicate along with nitrate and phosphate during both the study periods. During November, an increase in phytoplankton biomass was noticed in all pollutant-added samples, except municipal sewage, due to the limitation of growth by nitrate. On the other hand, an increase in biomass and abundance of phytoplankton was observed in all pollutant-added samples, except for aerosol, during January. Increase in phytoplankton abundance associated with decrease in biomass was observed in aerosol-added sample due to co-limitation of silicate and phosphate during January. A significant response of Thalassiothrix sp. was observed for industrial effluent-added sample during November, whereas Chaetoceros sp. and Skeletonema sp. increased significantly during January. Higher increase in phytoplankton biomass was observed during November associated with higher availability of silicate in the coastal waters in January. Interestingly, an increase in the contribution of dinoflagellates was observed during January associated with low silicate in the coastal waters, suggesting that the concentration of silicate in the coastal waters determines the response of the phytoplankton group to pollutant inputs. This study suggested that silicate concentration in the coastal waters must be considered, in addition to the coastal currents, while computing dilution factors for the release of pollutants to the coastal ocean to avoid occurrence of unwanted phytoplankton blooms.

Dynamic adaptation of phytoplankton vertical migration to changing grazing and nutrient conditions

AbstractDespite the ubiquitous occurrence of vertical migration of phytoplankton its quantitative significance remains poorly known. We eliminated a dense Daphnia population in a pond by introducing whitefish fingerlings, and assessed the effects on the vertical migration of dominating motile phytoplankton. At the highest abundance of Daphnia, cryptophytes reduced grazing losses by staying in the hypolimnion day and night, but Mallomonas species armoured by silica bristles remained in the epilimnion. After the fish introduction, phytoplankton was released from Daphnia grazing pressure, allowing cryptophytes to occur in the epilimnion also at noon. At the same time, increased phytoplankton biomass exacerbated the nutrient depletion. Cryptophytes compensated for that by migrating into the anoxic hypolimnion, whereupon their growth rates increased. The collapse of Daphnia was also associated with a temporary increase in nutrient regeneration by enzyme activities and decreases in total nutrient concentration and bacterial biomass in the whole water column. Our results show that cryptophytes can dynamically modify their vertical migration to balance between the exploitation of various nutrient resources and the risk of becoming eaten. Hypolimnetic nutrient resources can be quantitatively more important for phytoplankton than previously assumed.

Clams on stilts: a phytoplankton bioassay investigating effects of wastewater effluent amendments and Corbicula fluminea grazing

Shallow-water habitats are being restored in the Sacramento-San Joaquin River Delta with the goal of enhancing phytoplankton production and food availability for higher trophic levels. However, elevated grazing pressure from the non-native freshwater clam Corbicula fluminea and localized depletions of dissolved inorganic nitrogen may limit phytoplankton biomass accumulation in restored habitats. To evaluate interactions between nutrients and grazing on phytoplankton productivity and biomass accumulation, Sacramento River water high or low in phytoplankton biomass was amended with wastewater effluent, presence of C. fluminea, or both, in 48 h in situ incubations. We measured changes in chl a concentration, phytoplankton community composition, and photosynthetic efficiency as well as carbon and nitrogen uptake rates as indicators of phytoplankton responses. Diatoms dominated phytoplankton communities before and after incubation. Chl a concentrations increased 0.7 and 7.4 times in the high and low phytoplankton biomass controls, respectively, and 4.5 and 14 times in the high and low phytoplankton biomass effluent-added treatments, respectively. In the clam treatments, chl a accumulation was suppressed to near zero regardless of effluent additions or initial phytoplankton biomass. In treatments with clams and effluent combined, phytoplankton photosynthetic efficiency was nearly 50% lower than in the effluent-only treatments, suggesting phytoplankton were stressed in the presence of clams. This experiment demonstrated that the presence of clams can prevent the accumulation of phytoplankton biomass, both directly by clam filtering and indirectly by depressing phytoplankton photosynthetic efficiency and rate of growth. We recommend that future wetland restoration projects promoting increased phytoplankton biomass assess clam settlement likelihood as well as nutrient availability.

Nutrient enrichment alters phytoplankton biomass and composition via silicon limitation

Despite management efforts, anthropogenic nutrient enrichments continue to enhance phytoplankton blooms worldwide. Release of nitrogen and phosphorus compounds not only provides surplus of nutrients but also disbalances their stoichiometry. Declines in the relative availability of dissolved silicon might induce limitation in diatoms, major primary producers with silicified shells. We studied experimentally how nutrient enrichment and resulting decline in dissolved silicon to nitrogen ratios (Si:N) affect the structure and functioning of natural plankton communities. Nitrate was added to create a range of Si:N ratios and phosphate was supplied in Redfield ratio to nitrogen. We also manipulated copepod abundance to understand the top-down effects on communities experiencing nutrient enrichment. Nitrogen and phosphorus additions resulted in a steep phytoplankton biomass increase, followed by a post-bloom decline. Phytoplankton bloom biomass was higher in high nitrogen treatments but during the post-bloom period this trend switched. Biomass was sustained longer in high Si:N treatments, indicating that silicon limitation terminates the bloom. Many diatom species did not benefit from nitrogen and phosphorus enrichment and diatom dominance ceased below Si:N of 0.4:1. Under high grazing pressure, silicate was taken up faster suggesting that silicification is important in diatom defense. Copepods shaped plankton communities via feeding on dinoflagellates, chlorophytes and the diatom Skeletonema costatum but there was no significant effect of nitrogen and phosphorus enrichment on copepod abundance. Our results, combined with previous studies, show that while nutrient concentrations define the total phytoplankton bloom biomass, resource ratios are important in sustaining biomass and determining community structure and composition.

Experimental warming promotes CO2 uptake but hinders carbon incorporation toward higher trophic levels in cyanobacteria-dominated freshwater communities

Shallow freshwaters can exchange large amounts of carbon dioxide (CO2) with the atmosphere and also store significant quantities of carbon (C) in their sediments. Current warming and eutrophication pressures might alter the role of shallow freshwater ecosystems in the C cycle. Although eutrophication has been widely associated to an increase in total phytoplankton biomass and particularly of cyanobacteria, it is still poorly understood how warming may affect ecosystem metabolism under contrasting phytoplankton community composition. We studied the effects of experimental warming on CO2 fluxes and C allocation on two contrasting natural phytoplankton communities: chlorophytes-dominated versus cyanobacteria-dominated, both with a similar zooplankton community with a potentially high grazing capacity (i.e., standardized density of large-bodied cladocerans). The microcosms were subject to two different constant temperatures (control and +4 °C, i.e., 19.5 vs 23.5 °C) and we ensured no nutrient nor light limitation. CO2 uptake increased with warming in both communities, being the strongest in the cyanobacteria-dominated communities. However, only a comparatively minor share of the fixed C translated into increased phytoplankton (Chl-a), and particularly a negligible share translated into zooplankton biomass. Most C was either dissolved in the water (DIC) or sedimented, the latter being potentially available for mineralization into DIC and CO2, or methane (CH4) when anoxic conditions prevail. Our results suggest that C uptake increases with warming particularly when cyanobacteria dominate, however, due to the low efficiency in transfer through the trophic web the final fate of the fixed C may be substantially different in the long run.

Effects of pH Changes on Phytoplankton Biomass

A phytoplankton was incubated for a week to determine the effects of pH changes on its biomass growth in nutrient enrichment incubation samples. In this experiment, a general increase in phytoplankton biomass was observed after 24 hours of incubation, with a similar growth pattern in all incubation samples. By comparing with the same pH range (7.0 to 8.0), Pulau Pangkor (PP) incubation samples achieved the peaks earlier compared to Pulau Redang (PR) samples, although they had a higher increment in biomass. Meanwhile, in extreme pH (4.0 and 9.0) incubation samples, the phytoplankton biomass was observed to thrive well. This present study suggests that the phytoplankton community in Pulau Redang and Pulau Pangkor waters is able to survive in a wide range of pH levels, and the change in ocean pH has no vital impacts on the phytoplankton based on the short-term experiment.

Controls and significance of priming effects in lake sediments

AbstractWarming and eutrophication influence carbon (C) processing in sediments, with implications for the global greenhouse‐gas budget. Temperature effects on sedimentary C loss are well understood, but the mechanism of change in turnover through priming with labile organic matter (OM) is not. Evaluating changes in the magnitude of priming as a function of warming, eutrophication, and OM stoichiometry, we incubated sediments with 13C‐labeled fresh organic matter (FOM, algal/cyanobacterial) and simulated future climate scenarios (+4°C and +8°C). We investigated FOM‐induced production of CH4 and microbial community changes. C loss was primed by up to 17% in dominantly allochthonous sediments (ranging from 5% to 17%), compared to up to 6% in autochthonous sediments (−9% to 6%), suggesting that refractory OM is more susceptible to priming. The magnitude of priming was dependent on sediment OM stoichiometry (C/N ratio), the ratio of fresh labile OM to microbial biomass (FOM/MB), and temperature. Priming was strongest at 4°C when FOM/MB was below 50%. Addition of FOM was associated with activation and growth of bacterial decomposers, including for example, Firmicutes, Bacteroidetes, or Fibrobacteres, known for their potential to degrade insoluble and complex structural biopolymers. Using sedimentary C/N > 15 as a threshold, we show that in up to 35% of global lakes, sedimentation is dominated by allochthonous rather than autochthonous material. We then provide first‐order estimates showing that, upon increase in phytoplankton biomass in these lakes, priming‐enabled degradation of recalcitrant OM will release up to 2.1 Tg C annually, which would otherwise be buried for geological times.

Tropical cyclone effects enhance limnetic plankton trophic‐level relationships in a subtropical oligotrophic freshwater ecosystem

Abstract Tropical cyclones (TCs), as natural extreme weather events, alter plankton and hydrological environments, affecting the stability of biological processes in freshwater ecosystems, and such TC effects vary with water depths. Previous studies have found increased phytoplankton biomass resulting from TC effects has been observed, leading to potential strong grazing of zooplankton and enhanced plankton trophic‐level relationships. However, this remains understudied, particularly under in situ conditions. Using a zooplankton to phytoplankton (ZB/PB) ratio to represent the plankton trophic‐level relationship, we estimated the ZB/PB ratios at various depth intervals, including surface (2 m depth) and euphotic (depths between 0 and 20 m) depths and depth layer 0–50 m (depths between 0 and 50 m), in a subtropical deep oligotrophic freshwater ecosystem from 2012 to 2015 to understand how TC effects would influence changes in the ZB/PB ratio variations. TCs affected the surface and euphotic ZB/PB ratios but not those at the depth layer 0–50 m. The TC durations had an initially negative and then positive impact on the surface ZB/PB ratio, indicating that slow‐moving TCs might restructure surface plankton trophic‐level relationships. The water temperature and nutrient dynamics during the TC weeks showed the highest correlations with the ZB/PB ratios at the surface and euphotic depths. The combined environmental effects influenced the ZB/PB ratios at the surface and euphotic depths during the TC weeks, with 65.1% and 72.2% of the total variations explained in the multivariate regressions, respectively. There were greater impacts of TCs in shallow water (surface and euphotic depth) than in deep water. Aquatic food chains may be unexpectedly vulnerable to natural extreme weather events, such as TCs, and continuous assessments of food chain dynamics are necessary to better manage potential risks from natural extreme weather events in freshwater ecosystems.

Enhanced Phosphate Consumption Stimulated by Nitrogen Fixation Within a Cyclonic Eddy in the Northwest Pacific

AbstractMesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region.

Assessing drivers of estuarine pH: A comparative analysis of the continental U.S.A.'s two largest estuaries

AbstractIn estuaries, local processes such as changing material loads from the watershed and complex circulation create dynamic environments with respect to ecosystem metabolism and carbonate chemistry that can strongly modulate impacts of global atmospheric CO2 increases on estuarine pH. Long‐term (> 20 yr) surface water pH records from the USA's two largest estuaries, Chesapeake Bay (CB) and Neuse River Estuary‐Pamlico Sound (NRE‐PS) were examined to understand the relative importance of atmospheric forcing vs. local processes in controlling pH. At the estuaries’ heads, pH increases in CB and decreases in NRE‐PS were driven primarily by changing ratios of river alkalinity to dissolved inorganic carbon concentrations. In upper reaches of CB and middle reaches of the NRE‐PS, pH increases were associated with increases in phytoplankton biomass. There was no significant pH change in the lower NRE‐PS and only the polyhaline CB showed a pH decline consistent with ocean acidification. In both estuaries, interannual pH variability showed robust, positive correlations with chlorophyll a (Chl a) during the spring in mid to lower estuarine regions indicative of strong control by net phytoplankton production. During summer and fall, Chl a and pH negatively correlated in lower regions of both estuaries, given a shift toward heterotrophy driven by changes in phytoplankton community structure and increases in the load ratio of dissolved inorganic nitrogen to organic carbon. Tropical cyclones episodically depressed pH due to vertical mixing of CO2 rich bottom waters and post‐storm terrestrial organic matter loading. Local processes we highlight represent a significant challenge for predicting future estuarine pH.

Eutrophication assessment and environmental management perspectives of Tóbari: an arid subtropical coastal lagoon of the Gulf of California.

Coastal lagoons are vulnerable to eutrophication processes. In this study, we evaluate the eutrophication process in the restricted, arid subtropical Tóbari coastal lagoon, located in the eastern coast of the Gulf of California, where the main source of nutrient inputs and other pollutants is agricultural wastewater from the Yaqui Valley. The Assessment of Estuarine Trophic Status (ASSETS) model and the Trophic State Index (TRIX) were used to evaluate eutrophication. Overall, ASSETS showed that the Tóbari lagoon has a moderate eutrophication process, with seasonal symptoms of hypoxia, increased phytoplankton biomass, dominance of macroalgae (indicative of nutrient enrichment), and blooms development of potentially harmful algae species. The TRIX showed that the lagoon is mesotrophic most of the year. Challenges of environmental management detected correspond to reducing the input of nutrients and others contaminants from anthropic sources: agriculture, shrimp farming, livestock, and urban zones.

Global variability of high-nutrient low-chlorophyll regions using neural networks and wavelet coherence analysis

Abstract. We examine 20 years of monthly global ocean color data and modeling outputs of nutrients using self-organizing map (SOM) analysis to identify characteristic spatial and temporal patterns of high-nutrient low-chlorophyll (HNLC) regions and their association with different climate modes. The global nitrate-to-chlorophyll ratio threshold of NO3 : Chl > 17 (mmol NO3 mg Chl−1) is estimated to be a good indicator of the distribution limit of this unproductive biome that, on average, covers 92 × 106 km2 (∼ 25 % of the ocean). The trends in satellite-derived surface chlorophyll (0.6 ± 0.4 % yr−1 to 2 ± 0.4 % yr−1) suggest that HNLC regions in polar and subpolar areas have experienced an increase in phytoplankton biomass over the last decades, but much of this variation, particularly in the Southern Ocean, is produced by a climate-driven transition in 2009–2010. Indeed, since 2010, the extent of the HNLC zones has decreased at the poles (up to 8 %) and slightly increased at the Equator (< 0.5 %). Our study finds that chlorophyll variations in HNLC regions respond to major climate variability signals such as the El Niño–Southern Oscillation (ENSO) and Meridional Overturning Circulation (MOC) at both short (2–4 years) and long (decadal) timescales. These results suggest global coupling in the functioning of distant biogeochemical regions.

Source, composition, and reactivity of particulate organic matter along the Changjiang Estuary salinity gradient and adjacent sea

The sources, distribution, and fate of particulate organic matter (POM) in estuaries are dynamic and complex, influenced by highly intensive human activities and high productivity. In this study, water samples were collected along the Changjiang Estuary salinity gradient and adjacent sea (CEAS) in February and May 2017. Particulate organic carbon (POC), particulate nitrogen (PN), the δ13C isotope values and major biochemical constituent (total particulate amino acids, TPAA) were measured. The concentrations of POC, PN, and TPAA showed an overall decreasing trend from the river end-member to the open sea; however, their maximum always occurred around the turbidity maximum zone (TMZ). Concentrations of POC and TPAA showed a negative correlation with salinity and a positive correlation with chlorophyll a, indicating that the variation in POM concentrations and composition was mainly controlled by both terrigenous input and in situ phytoplankton production. The δ13C values gradually increased from the river mouth to the open sea in both winter and spring, in contrast to the molar C/N, reflecting the transition from terrestrial POC to phytoplankton-derived fresh POC with increasing salinity. Major biochemical indicators of TPAA/POC (%) and the degradation index (DI), showed a gradual shift towards more bioactive POM with increasing salinity in spring, although low TPAA/POC (%) values appeared within the TMZ. In spring, POC reactivity was higher than in winter. The proportions of glycine (Gly) and serine (Ser) were higher in winter, indicating that POM had suffered extensive degradation. Based on a two end-member mixing model, the contribution of marine POC in spring (53 ± 14%) was significantly greater than in winter (39 ± 19%), indicating that phytoplankton-derived POM was dominant in spring, associated with the increase in phytoplankton biomass from winter to spring. Based on mass balance, a box model showed evidence of a net POC sink over the Changjiang estuary and its adjacent East China Sea shelf in both winter and spring, with a net POC budget of 20.49 ± 7.01 and 15.87 ± 6.57 kmol s−1, respectively. Results illustrate that the spatio-temporal distribution of POM varies distinctively and will further affect the variability in its composition and reactivity in the CEAS.

Role of physical processes in regulating the biological responses in the Eastern Arabian Sea during the late southwest monsoon

Understanding physical properties and associated biological responses in the marine ecosystem is critical for fisheries and assessing the ocean’s role in determining the fate of anthropogenic carbon dioxide emissions. This study reports the different roles of southwest monsoon (SWM) upwelling, oceanic fronts, and mesoscale eddies in supplying nutrients and fueling phytoplankton production in the eastern Arabian Sea (EAS). In-situ measurements of physical parameters along with the distribution of biogeochemical parameters in the 100 m water column over a wide latitudinal transect from 8°N to 21°N (Station II-1 to II-14 at 1° interval) along the 68°E were carried out during the late SWM (September 11–19) of 2017. We found distinct features in the study area from the in-situ measurements and satellite data. Primarily, oceanic fronts along with higher instabilities caused vertical mixing at stations II-6, and II-10 (13°N and 17°N). Secondly, eddy-induced lateral advection of high productive water from the Western Arabian Sea (WAS) resulted in high phytoplankton biomass at stations II-7, and II-8 (14°N and 15°N), which led to high concentrations of pigments (chlorophyll-a (Chl-a) and fucoxanthin). SWM upwelling induced higher phytoplankton biomass at northern stations II-13 and II-14 and is evident from the higher total organic carbon (TOC), high total bacterial count (TBC) and Chl-a concentration (up to 0.371μg/L at II-14). The difference in time lag for vertical mixing at II-6 and II-10 are responsible for the contrasting biological responses in these stations. The present study showed that along with SWM upwelling, eddy-induced lateral advection from WAS and oceanic fronts are also increasing phytoplankton biomass in the EAS. The present study suggests that the high phytoplankton biomass prompted by upwelling, oceanic fronts, and mesoscale eddies can influence CO2 sequestration and the prevailing subsurface oxygen minimum zone in the EAS.

Health and Safety Effects of Airborne Soil Dust in the Americas and Beyond

AbstractRisks associated with dust hazards are often underappreciated, a gap between the knowledge pool and public awareness that can be costly for impacted communities. This study reviews the emission sources and chemical, physical, and biological characteristics of airborne soil particles (dust) and their effects on human and environmental health and safety in the Pan‐American region. American dust originates from both local sources (western United States, northern Mexico, Peru, Bolivia, Chile, and Argentina) and long‐range transport from Africa and Asia. Dust properties, as well as the trends and interactions with criteria air pollutants, are summarized. Human exposure to dust is associated with adverse health effects, including asthma, allergies, fungal infections, and premature death. In the Americas, a well‐documented and striking effect of soil dust is its association with Coccidioidomycosis, commonly known as Valley fever, an infection caused by inhalation of soil‐dwelling fungi unique to this region. Besides human health, dust affects environmental health through nutrients that increase phytoplankton biomass, contaminants that diminish water supply and affect food (crops/fruits/vegetables and ready‐to‐eat meat), spread crop and marine pathogens, cause Valley fever among domestic and wild animals, transport heavy metals, radionuclides and microplastics, and reduce solar and wind power generation. Dust is also a safety hazard to road transportation and aviation, in the southwestern US where blowing dust is one of the deadliest weather hazards. To mitigate the harmful effects, coordinated regional and international efforts are needed to enhance dust observations and prediction capabilities, soil conservation measures, and Valley fever and other disease surveillance.

Long‐term phytoplankton community dynamics in Lake Stechlin (north‐east Germany) under sudden and heavily accelerating eutrophication

Abstract Climate warming and legacy phosphorus (P) in sediments are two main factors counteracting current efforts to prevent eutrophication of temperate freshwater lakes and reservoirs. Climate warming impacts lacustrine thermal regimes, accelerates harmful cyanobacterial growth, and increases the probability of periodic anoxic bottom conditions. Legacy P in sediments becomes influential over eutrophication because of its reactivation under anoxic conditions. Lake Stechlin is a deep glacial lake of the Baltic Lake area, Germany in which artificial external P loading has been effectively blocked for more than 3 decades. The lakes trophic status was oligotrophic, until roughly 2 decades ago when it accelerated toward higher trophic levels. Here we analyse the temporal dynamics of Lake Stechlin phytoplankton biomass and composition in relation to chemical and hydrological covariates, based on monitoring data from 1994 to 2019 at sampling frequency of c. 27 samples per year. Our results show an assemblage change toward cyanobacterial dominance that took place at several temporal scales. Intra‐annual compositional change became more pronounced over the study period, mainly because of changes during the autumn and winter seasons. Results from a structural break point analysis of the phytoplankton time series with covariates, and from a hierarchical cluster analysis show that major changes occurred in 2007–2011. An analysis of early warning indicators did not unambiguously support this period as a critical transition. The change rather occurred as a rapid succession of major events during these years. Results from convergent cross‐mapping causality tests are consistent with the hypothesis that increasing relative water column stability may drive eutrophication, and additionally, that the intra‐annual timing of the biomass change plays an important role in the lakes P loading. Although quantitative data on internal P loading are unavailable, internal P mobilisation from sediments appears to be the most plausible mechanism behind the increased phytoplankton biomass. Lake Stechlin, therefore, is an example of the challenges that legacy P poses to the management of freshwater ecosystems in the context of climate warming, even when external P sources are effectively blocked, and local protective measures are in place.

Top-down and bottom-up control of phytoplankton communities by zebra mussels Dreissena polymorpha (Pallas, 1771)

Zebra mussels (ZM), Dreissena polymorpha (Pallas, 1771), are one of the most aggressive invasive species. ZM have a strong bidirectional impact on phytoplankton because of their high nutrient excretion rates and high grazing pressure. Furthermore, the interactions between excretion and selective grazing are related to the trophic status of a waterbody and could cause unpredictable changes in phytoplankton. We performed three types of experiments: (i) bottom-up where we measured ZM excretion rates; (ii) top-down where we analyzed the effects of ZM on phytoplankton taxonomic structure via grazing in different trophic conditions; (iii) mesocosm experiment where we estimated the combined top-down and bottom-up effects of ZM on phytoplankton assemblages under different trophic conditions. Our first experiment confirmed high excretion rates of dissolved nutrients (PO43− and NH4+) and dissolved organic carbon (DOC) by ZM. The other experiments revealed selective grazing by ZM, where diatoms were mostly selectively rejected, while green algae were preferred. In the top-down experiment, ZM decreased the total biomass of phytoplankton, while in mesocosm experiments where top-down and bottom-up controls acted simultaneously, we observed increased phytoplankton biomass mainly through increases in filamentous green algae. Our experiments show that ZM can influence phytoplankton through a combination of bottom-up and top-down effects that vary with trophic state.

Deoxygenation, Acidification and Warming in Waquoit Bay, USA, and a Shift to Pelagic Dominance

Coastal nutrient pollution, or eutrophication, is commonly linked to anthropogenic influences in terrestrial watersheds, where land-use changes often degrade water quality over time. Due to gradual changes, the management and monitoring of estuarine systems often lag environmental degradation. One example can be found at the Waquoit Bay National Estuarine Research Reserve, where we developed an analysis framework to standardize and analyze long-term trends in water quality and submerged vegetation data from monitoring programs that began in the 1990s. These programs started after the nearly complete loss of historically extensive Zostera marina (eelgrass) meadows throughout the estuary. Recently, eelgrass only persisted in small, undeveloped sub-embayments of the estuary, with conservative declines of over 97% in areal coverage. Over the past 2 decades, the average deoxygenation, acidification, and warming were −24.7 µmol O2 kg−1 (−11%), 0.006 µmol H+ kg−1 (+ 34%), and 1.0 °C (+ 4%), respectively. Along with the loss of eelgrass, there was also a decline in macroalgal biomass over 3 decades, resulting in a system dominated by pelagic metabolism, indicated by a 71% increase in water column chlorophyll a concentrations since 2009. This recent increase in phytoplankton biomass, which is highly mobile and transported throughout the estuary by tides, has resulted in recent degradation of isolated embayments despite their lower nutrient loads. This shift toward pelagic dominance in Waquoit Bay may indicate that other eutrophic and warming estuaries may also shift toward pelagic dominance in the future, as the Northeastern US is one of the fastest warming regions across the world.

How does aquaculture activity affect phytoplankton functional groups in Gaoyou Lake, China

Aquaculture in lakes is an important human activity that could accelerate eutrophication. Its impacts on aquatic ecosystems have attracted extensive attention. This study was carried out in Gaoyou Lake, China. We investigated environmental variables and phytoplankton in areas with different aquaculture intensities over four seasons. Marked differences in diversity, biomass and phytoplankton community structure were observed across areas. The 59 phytoplankton taxa identified could be divided into 20 functional groups (PFGs), of which groups B, D, E, J, MP, P, S1, W1 and Y were dominant PFGs in different areas. Total phosphorus (TP), ratio of nitrogen and phosphorus (TN/TP) and chemical oxygen demand (CODMn) were important driving factors for spatial differences in phytoplankton functional groups. Aquaculture altered the phytoplankton community structure, leading to a significant increase in phytoplankton biomass and diversity, especially cyanobacterial biomass. However, phytoplankton biomass and diversity could be significantly reduced and restored to natural levels after the cessation of aquaculture. This study clarified the effects of aquaculture activity on the phytoplankton community, thereby providing fundamental information for better understanding the impacts of aquaculture on lake eutrophication.

Browning-induced changes in trophic functioning of planktonic food webs in temperate and boreal lakes: insights from fatty acids

The effects of lake browning on trophic functioning of planktonic food webs are not fully understood. We studied the effects of browning on the response patterns of polyunsaturated fatty acids and n−3/n−6 ratio in seston and compared them between boreal and temperate lakes. We also compared the regional differences and the effects of lake browning on the reliance of zooplankton on heterotrophic microbial pathways and the mass fractions of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in zooplankton. Lake browning was associated with increasing phytoplankton biomass and concentrations of EPA and DHA in both temperate and boreal lakes, but the seston n−3/n−6 ratio was lower in temperate than boreal lakes, most likely due the differences in phytoplankton community composition. The browning-induced increase in phytoplankton biomass was associated with increased reliance of zooplankton on a heterotrophic microbial pathway for both cladocerans and copepods in boreal and temperate lakes. This increased reliance on the heterotrophic microbial diet was correlated with a decrease in the EPA and DHA mass fractions in temperate copepods and a decrease in the n−3/n−6 ratio in boreal cladocerans and copepods. Our results indicate that although phytoplankton responses to lake browning were similar across regions, this did not directly cascade to the next trophic level, where zooplankton responses were highly taxa- and region-specific. These results indicate that lake browning should be considered as an overarching moderator that is linked to, e.g., nutrient increases, which have more immediate consequences on trophic interactions at the phytoplankton–zooplankton interface.