Increase In Phytoplankton Abundance 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.

Effects of extreme rainfall events on phytoplankton community in a subtropical eutrophic lake: a mesocosm experiment

The impact of global climate change has led to an increase in extreme rainfall events, causing fluctuations in terrigenic inputs that significantly affect aquatic communities. Lake Changhu, the third-largest freshwater lake in Hubei Province, plays a crucial role as a reservoir of aquatic germplasm resources in the middle reaches of the Yangtze River. In recent years, the lake has experienced a series of extreme rainfall events. In response to recent extreme rainfall events, a 42-day mesocosm experiment was conducted to understand the effect of terrigenic inputs on various aspects of phytoplankton in Lake Changhu, such as species composition, abundance, biomass, diversity, community turnover rates, resource use efficiency, and stability. The experiment involved the application of different terrigenic treatments, including a control group (CK, using lake water), low terrigenic input (LT), medium terrigenic input (MT), and high terrigenic input (HT). The results showed a noticeable shift in phytoplankton composition from a co-dominated state of Chlorophyta and Cyanobacteria in the CK treatment to a Cyanobacteria-dominated state in the other terrigenic treatments. Furthermore, the terrigenic inputs increased phytoplankton abundance, community turnover rates, diversity, and resistance. Comparatively, the diversity index of phytoplankton increased by 82.61%, 73.83%, and 70.41% in the LT, MT, and HT treatments, respectively, in contrast to the CK treatment. However, phytoplankton abundance decreased by 6.99%, 15.55%, and 14.76% in the LT, MT, and HT treatments. Additionally, the resource use efficiency decreased by 1.94%, 5.16%, and 14.19% in the LT, MT, and HT treatments, respectively, compared to the CK treatment. These findings provide valuable insights into monitoring and managing the water ecology in Lake Changhu, offering a scientific basis for implementing effective management strategies.

Anthropogenic forcing leads to an abrupt shift to phytoplankton dominance in a shallow eutrophic lake

Abstract The timing and causes of lake eutrophication are often obscured when multiple anthropogenic disturbances coincide in space and time. This issue is particularly problematic for shallow lakes in arid regions that experience strong climatic forcing which alters lake hydrology and water levels, and further conflates causal drivers. We used Utah Lake (Utah, U.S.A.) as a model system to examine how natural hydrological variability and anthropogenic forcing influence ecosystem structure of large shallow lakes in arid climates. Paleolimnological analyses of sedimentary biogeochemistry, pigments, DNA, and morphological fossils were used to identify shifts in primary production and evaluate the relative influence of regional climate‐driven hydrological variability and of humans on ecosystem structure. Sediment cores revealed that the phase prior to non‐indigenous settlement included numerous macrophyte and gastropod remains, sedimentary DNA from plants, low organic matter, and low algal production. An abrupt transition occurred in the late 19th century concomitant with agricultural and urban expansion and the introduction of common carp, which was characterised by a loss of macrophytes and an increase in phytoplankton abundance as indicated by sedimentary DNA and pigment concentrations. A further shift to increased cyanobacteria occurred c. 1950 when exponential population growth increased wastewater influx, as recorded by sedimentary δ15N values. Taken together, our data demonstrate that the current eutrophic state of Utah Lake is a function of anthropogenic forcing rather than natural climate‐driven hydrological fluctuations. Furthermore, large lakes in arid regions can exhibit similar patterns of abrupt ecosystem change between alternate states as those observed in northern temperate/boreal and subtropical ecosystems.

Scenarios of temporal environmental alterations and phytoplankton diversity in a changing bay in the East China Sea

In the context of global change, the stressors of warming and eutrophication have significant ecological implications in coastal waters. In order to examine the diversity of phytoplankton and its relationship with water quality, we conducted a survey of phytoplankton community compositions and their correlation with environmental changes over four seasons in a eutrophic bay located in the East China Sea. Through a systematic analysis, we identified diatoms and dinoflagellates as the primary dominant groups, with the species Skeletonema costatum, Skeletonema marinoi, Biddulphia sinensis, Thalassiosira eccentrica, Leptocylindrus danicus, Coscinodiscus oculus-iridis, Coscinodiscus jonesianus, and Chaetoceros knipowitschi as the most abundant species in all seasons. Significant seasonal alterations were observed in both environmental settings and phytoplankton species richness, dominance, and abundance. The phytoplankton community varied in its response to diverse aquatic environments and was principally affected by temperature, silicic acid concentrations, and suspended solids. Elevated temperatures were found to promote an increase in phytoplankton abundance. However, no clear evidence of diatom and dinoflagellate succession in relation to N:P ratio was observed across seasons. Water quality analysis illustrated that the majority of the study area exhibited a mid-eutrophic with severe organic pollution. The abundance of phytoplankton was significantly influenced by eutrophication and organic pollution. The accelerated warming process related to coastal nuclear power plants and nutrient regime alterations significantly affect the temporal shift of the phytoplankton community. These findings contribute valuable insights into the effects of eutrophic environments on the structure of phytoplankton communities in coastal aquatic systems.

Pathways linking nutrient enrichment, habitat structure, and parasitism to host-resource interactions.

Human activities simultaneously alter nutrient levels, habitat structure, and levels of parasitism. These activities likely have individual and joint impacts on food webs. Furthermore, there is particular concern that nutrient additions and changes to habitat structure might exacerbate the size of epidemics and impacts on host density. We used a well-studied zooplankton-fungus host-parasite system and experimental whole water column enclosures to factorially manipulate nutrient levels, habitat structure (specifically: mixing), and presence of parasites. Nutrient addition increased infection prevalence, density of infected hosts, and total host density. We hypothesized that nutrients, mixing, and parasitism were linked in multiple ways, including via their combined effects on phytoplankton (resource) abundance, and we used structural equation modeling to disentangle these pathways. In the absence of the parasite, both nutrients and mixing increased abundance of phytoplankton, whereas host density was negatively related to phytoplankton abundance, suggesting a mixture of bottom-up and top-down control of phytoplankton. In the presence of the parasite, nutrients still increased phytoplankton abundance but mixing no longer did, and there was no longer a significant relationship between host density and phytoplankton. This decoupling of host-resource dynamics may have resulted from reduced grazing due to illness-mediated changes in feeding behavior. Overall, our results show that the impact of one human activity (e.g., altered habitat structure) might depend on other human impacts (e.g., parasite introduction). Fortunately, carefully designed experiments and analyses can help tease apart these multifaceted relationships, allowing us to understand how human activities alter food webs, including interactions between hosts and their parasites and resources.

Correlation Analysis of Various Water Quality Parameters of the Tiru Reservoir, Maharashtra, India

The present study was conducted in the department of Aquatic Environment Management, College of Fishery Science, Udgir, District Latur, Maharashtra, India during August, 2017 to January, 2019 to investigate trophic status of the Tiru reservoir. Twenty (20) water quality parameters had been collected monthly from five sampling sites of Tiru reservoir, Udgir, Latur district, Maharashtra, India. Seasonal trends were analysed and discussed in this chapter. Pearson’s correlation coefficient (r) of eighteen physico-chemical parameters from five study locations were analyzed to know the pollution causing parameters. Water quality analysis showed that Total Phosphorus concentrations was highest in the summer, when reservoir water levels were lowest. Correlation between Total phosphorus and Chl-a was highest at 0.926. Eutrophication was mainly generated by phosphorus available in the water, which increases chlorophyll-a concentration (algal blooms). A positive correlation between temperature and DO might be possible because higher summertime water temperatures boost photosynthetic activity. SDD has shown a substantial negative connection with Chl-a throughout all seasons, since an increase in phytoplankton abundance yields a decrease in water transparency. The negative correlation between SDD and turbidity is primarily caused by the high levels of productivity in the summer, and high levels of suspended organic load due to surface runoff in the monsoon season. The majority of the time throughout the research period, Tiru reservoir was found in the eutrophic state, and in some cases even hyper-eutrophic.

Noctiluca Scintillans Distribution Largely Regulated by Phytoplankton Biomass in the East China Sea and Southern Yellow Sea

Noctiluca scintillans feeds on a large number of phytoplankton, including diatoms and dinoflagellates, and frequently forms a red tide in the East China Sea (ECS) and southern Yellow Sea (SYS). However, the spatiotemporal distribution pattern, controlling factors, and long-term change of N. scintillans in the ECS and SYS remain unclear. In the present study, we collected N. scintillans samples from the ECS and SYS throughout the four seasons of 2011. We sampled phytoplankton and environmental parameters simultaneously. The depth-integrated abundance (DIA) of N. scintillans was the highest and lowest in summer and winter, respectively. N. scintillans is distributed abundantly in eutrophic coastal waters and the Changjiang Estuary, which are characterized by high concentrations of phytoplankton and chlorophyll-a. A Spearman correlation test demonstrated that its DIA in the upper 30-m water column was generally more significantly associated with phytoplankton abundance and chlorophyll-a concentration than with temperature and salinity. The results of the generalized additive models revealed that chlorophyll-a concentration explained more of the variation in N. scintillans abundance than temperature and salinity throughout the year, particularly in warm seasons. These findings indicate that the seasonal and spatial changes of N. scintillans are largely regulated by phytoplankton biomass. Compared with the historical data from 1959 and 2002, the abundance of N. scintillans in the Changjiang Estuary increased considerably in 2011 with increasing phytoplankton abundance resulting from accelerated eutrophication and warming. These results clarify the controlling factors, red-tide formation mechanism, and changing trends associated with the N. scintillans in the ECS and SYS.

Climatology of Dust Deposition in the Adriatic Sea; a Possible Impact on Marine Production

AbstractAtmospheric mineral dust deposition plays an important role in providing nutrients to marine ecosystems. In this study, the climatology of dust deposition in the Adriatic Sea area was determined for the first time based on Modern‐Era Retrospective Analysis for Research and Applications, version 2 reanalysis from 1989 to 2019. The annual cycle of deposition exhibits two maxima: a stronger maxima in March–April and a weaker maxima in November. Wet deposition is a dominant process with a relative contribution to overall deposition from 67.35% to 88.53%. Deposition hot spots are along the Montenegrin coast and Otrant. The average contribution of dust deposition events (DDEs) is 16.5% (60.2 dy yr−1), with the strongest deposition during 1999–2009 and a positive trend in deposited mass during the study period. The effect of dust deposition on primary production is observed by a high level of oxygen saturation up to 250% (usually it is lower, up to 150%) in the stratified middle water layer (5–8 m) of the central Adriatic marine system (Rogoznica Lake) during an intense wet deposition episode. Such extreme values of oxygen saturation can be taken as an indication of biological activity related to an increase in phytoplankton abundance and activity, diatoms in particular.

Non-additive effects of foundation species determine the response of aquatic ecosystems to nutrient perturbation.

Eutrophication is a persistent threat to aquatic ecosystems worldwide. Foundation species, namely those that play a central role in the structuring of communities and functioning of ecosystems, are likely important for the resilience of aquatic ecosystems in the face of disturbance. However, little is known about how interactions among such species influence ecosystem responses to nutrient perturbation. Here, using an array (N=20) of outdoor experimental pond ecosystems (15,000L), we manipulated the presence of two foundation species, the macrophyte Myriophyllum spicatum and the mussel Dreissena polymorpha, and quantified ecosystem responses to multiple nutrient disturbances, spread over two years. In the first year, we added five nutrient pulses, ramping up from 10 to 50μg P/L over a 10-week period from mid-July to mid-October, and in the second year, we added a single large pulse of 50μg P/L in mid-October. We used automated sondes to measure multiple ecosystems properties at high frequency (15-minute intervals), including phytoplankton and dissolved organic matter fluorescence, and to model whole-ecosystem metabolism. Overall, both foundation species strongly affected the ecosystem responses to nutrient perturbation, and, as expected, initially suppressed the increase in phytoplankton abundance following nutrient additions. However, when both species were present, phytoplankton biomass increased substantially relative to other treatment combinations: non-additivity was evident for multiple ecosystem metrics following the nutrient perturbations in both years but was diminished in the intervening months between our perturbations. Overall, these results demonstrate how interactions between foundation species can cause surprisingly strong deviations from the expected responses of aquatic ecosystems to perturbations such as nutrient additions.

Shifts in the partitioning of benthic and pelagic primary production within and across summers in Lake Mývatn, Iceland

ABSTRACT The relative contributions of benthic and pelagic primary production affect ecosystem function, but studies documenting natural variation in the partitioning of production (i.e., autotrophic structure) are uncommon. This study examines autotrophic structure of shallow Lake Mývatn over 7 summers (2012–2018). We used routine measurements of benthic gross primary production (GPP), pelagic chlorophyll a concentrations, and pelagic production to estimate benthic and pelagic maximum productivity (P max) across summers. With these parameters and corresponding incident light and water clarity data, we estimated in situ benthic and pelagic GPP. Our results demonstrated substantial variation in Mývatn’s autotrophic structure within and across summers. Benthic GPP often exceeded pelagic GPP. However, periods of increased phytoplankton abundance were associated with increased light attenuation and, consequently, declines in benthic GPP. These effects were strongest in 3 summers with dense cyanobacteria blooms, in which the benthic fraction of total production declined from >95% to <20%. Reduced light levels over a 2-week period were associated with low benthic P max, implying that past shading by phytoplankton may decrease the photosynthetic potential of benthic producers. Moreover, variation in estimated benthic P max values can affect the point at which autotrophic structure shifts toward pelagic-dominated conditions. Overall, our study demonstrates that the balance between benthic and pelagic production can vary greatly at intra- and interannual scales because of changes in the photosynthetic capacity of both pelagic and benthic primary producers. Understanding natural variation in lake autotrophic structure may inform how benthic and pelagic production respond to ongoing and future environmental changes.

Continental-scale effects of phytoplankton and non-phytoplankton turbidity on macrophyte occurrence in shallow lakes.

Submerged macrophytes are key components of shallow lake biological communities, and their presence has been associated with a predominantly clear-water state. Conversely, lakes lacking macrophytes are often turbid with elevated phytoplankton abundance. One main mechanism that influences the presence or absence of submerged macrophytes is turbidity that reduces the light available to macrophytes. Increases in turbidity can be caused by increased phytoplankton abundance and by increased concentrations of suspended inorganic sediment and understanding the relative contributions of these two factors can inform efforts to manage the effects of increased turbidity on macrophyte occurrence. Here, a continental scale data set is analyzed to quantify the effects of macrophytes on turbidity that originates from phytoplankton and from non-phytoplankton sources (e.g., inorganic sediment). Effects of phytoplankton assemblage composition on turbidity are also estimated. Based on this model, illustrative examples of chlorophyll concentrations needed to maintain or restore macrophytes to shallow lakes are calculated, and the difference in the magnitude of these concentrations illustrates the stabilizing effect of macrophytes on lake condition.

Phaeocystis globosa Bloom Monitoring: Based on P. globosa Induced Seawater Viscosity Modification Adjacent to a Nuclear Power Plant in Qinzhou Bay, China

The mucus produced by the outbreak of Phaeocystis globosa in the adjacent waters of the Fangchenggang Nuclear Power Plant (FCGNPP) in China has blocked the entrance of the cooling water filtration system of the FCGNPP, and posed a threat to the safe operation of the FCGNPP. At present, there is no related research on whether the changes in seawater viscosity could be used as a new method for monitoring and providing early warning of P. globosa tide. During a complete red tide cycle, the temporal and spatial changes in the hydrological conditions (temperature, salinity, dissolved oxygen), chlorophyll-a (Chl-a), composition and abundance of phytoplankton, number and size of P. globose colonies, concentration of transparent exopolymer particles (TEP) and the seawater viscosity were measured in this study. The results indicate that there was an extremely significant negative correlation between the physical seawater viscosity ηT,S and temperature, and the correlation coefficient reached −0.998. The biological seawater viscosity ηBio was positively correlated with the number of P. globosa colonies, and the correlation coefficients is 0.747. Because the increase in phytoplankton abundance, especially during the outbreak of P. globosa and a large amount of mucus produced by the colonies could significantly increase seawater viscosity, we suggest that biological factors were the main reasons for the increase in seawater viscosity. The ηBio was completely consistent with the occurrence process of P. globosa bloom and could be used as a valuable index for P. globosa bloom monitoring.

Patterns of phytoplankton structure in response to environmental gradients in a macrotidal estuary of the Equatorial Margin (Atlantic coast, Brazil)

The structure of phytoplankton community was studied in a tropical macrotidal estuary (Paciência river estuary), which is socially and ecologically important because is surrounded by one of the most extensive mangrove forests of the world and plays a key role in the local economy, through artisanal fishing and bivalve extraction activities in natural environments. To understand the regulation mechanisms driving the phytoplankton dynamics and the key indicator species for monitoring ecological change, the study considered seasonal and spatial analysis of environmental and biological variables in thirteen sampling points during the three seasonal periods in 2017. The ecological patterns of phytoplankton were explored by Cluster Analysis and key species were selected by the indicator value method (IndVal). From 171 identified taxa, Bacillariophyta was the most representative division (128 taxa). The phytoplankton structure was well-defined by a salinity gradient governed by rainfall and river discharge. Thus, three scenarios related to seasonality and salinity gradient were identified: tidal river zone, mixing zone and coastal zone. During the rainy and transitional seasons, it was observed a spatial gradient of salinity, characterized by the gradual increase in phytoplankton abundance and biomass from the freshwater zone towards the coastal zone, following an opposite nutrient gradient. The highest salinity and light availability (low turbidity and total particulate matter) in the dry season favored a significant growth of phytoplankton (cell abundance and biomass). Thus, the key species selected by IndVal method was Discostella stelligera, Coscinodiscus oculus-iridis and Trieres sinensis in the rainy period, Nitzschia reversa, Thalassiosira sp. and Bellerochea horologicalis in the transitional period, and Protoperidinium sp., Thalassiosira subtilis, Cyclotella striata and Thalassiosira rotula in the dry period. Hence, this work contributes to the understanding of the macrotidal estuaries functioning, reinforcing the importance of phytoplankton monitoring for the identification of ecological changes.

Controls on Primary Productivity in the Eastern Equatorial Pacific, East of the Galapagos Islands, During the Penultimate Deglaciation

AbstractModern biogeochemical conditions of the Eastern Equatorial Pacific (EEP) region are characterized by high macronutrient concentrations but low phytoplankton abundance due to both iron and silicic acid limitation. Since primary producers significantly impact the global carbon cycle, paleoproductivity in relation to climate change and nutrient availability in this region has been a topic of a number of studies. However, the complex dynamics of this region, especially east of the Galapagos Islands, has led to some discrepancies when linking reconstructed paleoproductivity with potential mechanisms for higher primary productivity. Here we focus on reconstructing primary productivity of haptophyte algae and diatoms, as well as continental material input, sea surface salinity, and sea surface temperature, and compare these reconstructions with existing records for the period comprised between 150 and 110 ka (the penultimate deglaciation period) with the aim to understand the mechanisms that most significantly influence phytoplankton growth over the EEP region east of the Galapagos Islands. Our results suggest enhanced upwelling in the EEP system during the penultimate deglaciation and increased phytoplankton abundance mainly as the result of both the increasing influence of nutrient‐rich Southern Ocean sourced waters through the Equatorial Undercurrent and a higher input of iron through atmospheric deposition. The highest phytoplankton abundances recorded at the study site during the penultimate deglaciation also suggest that maximum input of nutrients might have occurred during the millennial‐scale event Heinrich Event 11 in the North Atlantic as a result of global atmospheric and oceanic reorganizations.

Differential Responses of Maximum Versus Median Chlorophyll‐a to Air Temperature and Nutrient Loads in an Oligotrophic Lake Over 31 Years

AbstractGlobally, phytoplankton abundance is increasing in lakes as a result of climate change and land‐use change. The relative importance of climate and land‐use drivers has been examined primarily for mesotrophic and eutrophic lakes. However, oligotrophic lakes show different sensitivity to climate and land‐use drivers than mesotrophic and eutrophic lakes, necessitating further exploration of the relative contribution of the two drivers of change to increased phytoplankton abundance. Here, we investigated how air temperature (a driver related to climate change) and nutrient load (a driver related to land‐use and climate change) interact to alter water quality in oligotrophic Lake Sunapee, New Hampshire, USA. We used long‐term data and the one‐dimensional hydrodynamic General Lake Model (GLM) coupled with Aquatic EcoDyanmics (AED) modules to simulate water quality. Over the 31‐year simulation, summer median chlorophyll‐a concentration was positively associated with summer air temperature, whereas annual maximum chlorophyll‐a concentration was positively associated with the previous 3 years of external phosphorus load. Scenario testing demonstrated a 2°C increase in air temperature significantly increased summer median chlorophyll‐a concentration, but not annual maximum chlorophyll‐a concentration. For both maximum and median chlorophyll‐a concentration, doubling external nutrient loads of total nitrogen and total phosphorus at the same time, or doubling phosphorus alone, resulted in a significant increase. This study highlights the importance of aligning lake measurements with the ecosystem metrics of interest, as maximum chlorophyll‐a concentration may be more uniquely sensitive to nutrient load and that typical summer chlorophyll‐a concentration may increase due to warming alone.

The Effect of Human Impact on the Water Quality and Biocoenoses of the Soft Water Lake with Isoetids: Lake Jeleń, NW Poland

Soft water lakes with isoetids (SLI) are ecosystems prone to degradation due to the low buffer capacity of their waters. One of the main threats resulting from human impact is eutrophication due to agriculture, catchment urbanization and recreational use. In this paper, changes in the water chemistry and transformation of biocoenoses of one of the largest Polish SLI, Lake Jeleń, over the past 30 years are presented. The lake is located within the borders of a city, and a significant part of its catchment is under agriculture and recreation use. The physicochemical (concentration of nutrients, organic matter, electrical conductivity, oxygen saturation and water pH) and biological parameters (macrophytes and phytoplankton) were measured in summer 1991, 2004, 2013 and 2018. Since the beginning of the 1990s, a gradual increase in the trophy of the lake has been observed as indicated by increased nutrient availability, deterioration of oxygen conditions and a decrease in water transparency. The alterations of water chemistry induce biological transformations, in particular, an increase in phytoplankton abundance (4-fold increase of biomass in epilimnion) as well as a gradual reduction in the range of the phytolittoral (from 10 to 6 m), a decrease in the frequency of isoetids, Lobelia dortmanna and Isoetes lacustris, and expansion of plant species characteristic for eutrophy.

Holocene organic geochemical record from the Western Indus continental shelf (northern Arabian Sea)

Primary production on the Western Indus continental shelf has been linked to the large quantities of nutrients delivered to the shelf by the Indus River. Multiple geochemical tracers and biomarker records, including stable isotopes (δ13C and δ15N), total organic carbon (TOC), total nitrogen (TN), molar carbon/nitrogen (C/N) ratio, the branched and isoprenoid tetraether (BIT) index, and glycerol dialkyl glycerol tetraether (GDGT), have been analyzed from the Indus-23AP sediment core recovered from the northern Arabian Sea. Our records show evidence of a mixture of marine and terrestrially derived organic matter (OM) during the last 14,000 years, as indicated by the C/N ratio, δ13C, δ15N, and the BIT index. The three sterol biomarkers (brassicasterol, dinosterol, and cholesterol) show concurrent enrichments during the last 3 millennia reflecting increased phytoplankton abundance because of increased Indus river discharge of nutrients during the summer monsoon. GDGT crenarchaeol enrichment is related to the BIT index. The TEX86-derived sea surface temperature (SST) record is shifted toward the summer season because Crenarchaeota are more abundant and active during periods of high primary production. SSTs indicate a long-term warming trend during the Holocene related to increasing winter insolation in the low latitudes northern Hemisphere.

Top-Down and Bottom-Up Controls on Microeukaryotic Diversity (i.e., Amplicon Analyses of SAR Lineages) and Function (i.e., Metatranscriptome Analyses) Assessed in Microcosm Experiments

The availability of high-throughput sequencing (HTS) has transformed our understanding of the diversity of microbial eukaryotes (i.e. protists) across diverse habitats. Yet relating this biodiversity to function remains a challenge, particularly in the context of microbial food webs. Here we perform a set of microcosm experiments to evaluate the impact of changing predator and prey concentrations on a marine protist community, focusing on SAR (Stramenopila, Alveolata, and Rhizaria) lineages. We combine an estimate of taxonomic diversity through analysis of SSU-rDNA amplicons with metatranscriptomics, a proxy for function. We assess changes in a community sampled from New England waters with varying concentrations of predators (copepods) and prey (phytoplankton less than 15µm in size). The greatest impact observed is on the diversity and function of the small plankton (2-10µm) community in the presence of high prey abundance (i.e. bloom conditions). Many SAR taxa in the nanosized fraction decrease with increasing phytoplankton abundance, while ciliates (from both nano- and microsized fraction) increase. A large number of transcripts and function estimates in the nanoplankton are downregulated during our simulated phytoplankton bloom. We also find evidence of an interaction between increasing phytoplankton and copepods on the microsized planktonic community, consistent with the hypothesis that phytoplankton and copepods exert bottom-up control and top-down control on the microsized protists, respectively. Together our analyses suggest community function (i.e. diversity of gene families) remains relatively stable, while the function at the species level (i.e. transcript diversity within gene families) show a substantial reduction of function under bloom conditions. Our study demonstrated that interactions within plankton food webs are complex, and that the relationships between diversity and function for marine microeukaryotes remain poorly understood.

Dynamic changes in dissolved organic matter composition in a Mountain Lake under ice cover and relationships to changes in nutrient cycling and phytoplankton community composition

We studied Bear Lake, a subalpine oligotrophic lake in Rocky Mountain National Park, Colorado USA, during the winter ice cover period of 2018. Our goal was to understand relationships between biogeochemical and biotic processes and the chemical quality of the dissolved organic matter (DOM), a major pool of organic carbon in all lakes. The lake was stratified with limited light availability and had a consistent oxycline at 7 m. Nutrient concentrations were low and the depletion of nitrogen species was related to increased phytoplankton abundance. The phytoplankton community became progressively dominated by a potentially mixotrophic dinoflagellate species, Gymnodinium sp., and a deep chlorophyll maximum developed above the hypolimnion. Changes in the chemical quality of DOM, as measured through high-resolution mass spectrometry, were associated with these shifts in nutrient and phytoplankton composition. The spectrometry data revealed a pool of organic compounds many of which contained phosphorus, nitrogen, and sulfur. This pool evolved through the winter, with phosphorus-containing compounds becoming more predominant in the upper oxic zone and sulfur-containing compounds becoming more predominant in the anoxic bottom waters. Reduced chemo diversity of the DOM pool in the surface waters developed through the winter. In the bottom waters, more unique compounds were produced through the winter, and the favorability of the DOM to support microbial respiration also increased. These observed differences in DOM molecular composition were not reflected in the spectroscopic signature of the DOM pool. Overall, our results show chemical changes in the DOM pool correlate to dynamic redox conditions, biogeochemical cycling, and biotic response to ice cover.

Modelling induced bank filtration effects on freshwater ecosystems to ensure sustainable drinking water production

Induced bank filtration (IBF) is a water abstraction technology using different natural infiltration systems for groundwater recharge, such as river banks and lake shores. It is a cost-effective pre-treatment method for drinking water production used in many regions worldwide, predominantly in urban areas. Until now, research concerning IBF has almost exclusively focussed on the purification efficiency and infiltration capacity. Consequently, knowledge about the effects on source water bodies is lacking. Yet, IBF interrupts groundwater seepage and affects processes in the sediment potentially resulting in adverse effects on lake or river water quality. Securing sufficient source water quality, however, is important for a sustainable drinking water production by IBF.In this study, we analysed the effects of five predicted mechanisms of IBF on shallow lake ecosystems using the dynamic model PCLake: declining CO2 and nutrient availability, as well as increasing summer water temperatures, sedimentation rates and oxygen penetration into sediments. Shallow lake ecosystems are abundant worldwide and characterised by the occurrence of alternative stable states with either clear water and macrophyte dominance or turbid, phytoplankton-dominated conditions. Our results show that IBF in most scenarios increased phytoplankton abundance and thus had adverse effects on shallow lake water quality. Threshold levels for critical nutrient loading inducing regime shifts from clear to turbid conditions were up to 80% lower with IBF indicating a decreased resilience to eutrophication. The effects were strongest when IBF interrupted the seepage of CO2 rich groundwater resulting in lower macrophyte growth. IBF could also enhance water quality, but only when interrupting the seepage of groundwater with high nutrient concentrations. Higher summer water temperatures increased the share of cyanobacteria in the phytoplankton community and thus the risk of toxin production. In relative terms, the effects of changing sedimentation rates and oxygen penetration were small. Lake depth and size influenced the effect of IBF on critical nutrient loads, which was strongest in shallower and smaller lakes. Our model results stress the need of a more comprehensive ecosystem perspective including an assessment of IBF effects on threshold levels for regime shifts to prevent high phytoplankton abundance in the source water body and secure a sustainable drinking water supply.