Changes In Phytoplankton Biomass Research Articles

Atmospheric deposition and river runoff stimulate the utilization of dissolved organic phosphorus in coastal seas

In coastal seas, the role of atmospheric deposition and river runoff in dissolved organic phosphorus (DOP) utilization is not well understood. Here, we address this knowledge gap by combining microcosm experiments with a global approach considering the relationship between the activity of alkaline phosphatases and changes in phytoplankton biomass in relation to the concentration of dissolved inorganic phosphorus (DIP). Our results suggest that the addition of aerosols and riverine water stimulate the biological utilization of DOP in coastal seas primarily by depleting DIP due to increasing nitrogen concentrations, which enhances phytoplankton growth. This “Anthropogenic Nitrogen Pump” was therefore identified to make DOP an important source of phosphorus for phytoplankton in coastal seas but only when the ratio of chlorophyll a to DIP [Log10 (Chl a / DIP)] is larger than 1.20. Our study therefore suggests that anthropogenic nitrogen input might contribute to the phosphorus cycle in coastal seas.

Temporal phytoplankton dynamics and environmental variables in four Ethiopian soda lakes

We investigated the spatio-temporal dynamics of phytoplankton composition, chlorophyll-a as a proxy for algal biomass, and abundance in relation to environmental parameters in four Ethiopian soda lakes: Arenguade, Beseka, Chittu, and Shala. Triplicate water samples were collected from each lake from January to December 2020, four times in different seasons. Lake Chittu had the highest chlorophyll-a concentration, followed by Lake Arenguade, Beseka and Shala. Chlorophyll-a concentrations generally increased during the post rainy and dry season. The results of LR models are high for lakes Arengude, Beseka and Chittu. Lakes Shala and Beseka had the highest number of phytoplankton taxa, with both taxa composition and abundance dominated by Bacillariophyceae. Cyanoprokaryota, particularly Limnospira fusiformis, predominated in the abundance of Lakes Arenguade and Chittu. Water temperature, Secchi depth, turbidity, electrical conductivity, soluble reactive phosphorus, nitrate and silica significantly influenced the phytoplankton community structure. Long-term trend analysis revealed changes in phytoplankton biomass and lake taxonomic composition. The alteration in phytoplankton biomass and species composition of the lakes could be attributed to three factors: (1) frequent high-velocity explosions conducted for seismological studies in the past. This impact caused a dramatic increase in lake level in the case of Lake Beseka leading to a drop in nutrient concentration; (2) climate change and (3) salt content. Overall, our findings suggest that phytoplankton composition, biomass, and abundance varied according to seasonal fluctuations, emphasizing the possible effects of anthropogenic and natural causes on their community structure.

Cladophora as ecological engineer: A new test from the largest lake of Qinghai-Tibet plateau with filamentous algal blooms

Filamentous algae blooms (FABs) have been increasing globally in recent years, and their presence can have both harmful and beneficial effects on aquatic ecosystems. As one of the most common FABs, Cladophora blooms have been reported in the lakes of the Qinghai-Tibet Plateau during the past few years. However, there have been few studies focused on how FABs impact other aquatic organisms, especially in alpine lakes since these are at the forefront of responding to global climate change. In this study, the phytoplankton communities in different regions of Qinghai Lake were profiled in different seasons using meta-barcode sequencing. The phytoplankton assemblages in areas with Cladophora blooms were compared to those without Cladophora. The phytoplankton community structure correlated with physicochemical properties including water temperature, electrical conductivity, nitrate, and the presence or absence of Cladophora blooms. The relative abundance of Bacillariophytes was found to be higher in zones with Cladophora blooms than in other regions. Significant seasonal changes in phytoplankton biomass and β diversity were observed in zones with Cladophora blooms. Growth and microbial degradation of Cladophora can change the pH, dissolved oxygen, secchi depth, and nitrate. Together with seasonal temperature and electrical conductivity changes, Cladophora growth can significantly impact the phytoplankton biomass, community dissimilarity and assembly process. These results showed that Cladophora plays a key role in littoral aquatic ecosystem ecology.

Atmospheric nourishment of global ocean ecosystems

Over the vast open ocean, vital nutrients for phytoplankton growth in the sunlit surface layer are largely provided through physical transport from deep waters, but some nutrients are also provided through atmospheric deposition of desert dust. The extent and magnitude of dust-mediated effects on surface ocean ecosystems have been difficult to estimate globally. In this work, we use global satellite ocean color products to demonstrate widespread responses to atmospheric dust deposition across a diverse continuum of phytoplankton nutritional conditions. The observed responses vary regionally, with some areas exhibiting substantial changes in phytoplankton biomass, whereas in other areas, the response reflects a change in physiological status or health. Climate-driven changes in atmospheric aerosols will alter the relative importance of this nutrient source.

Synoptic-to-intraseasonal atmospheric modulation of phytoplankton biomass in the inner sea of Chiloé, Northwest Patagonia (42.5°-43.5°S, 72.5°-74°W), Chile

The Inner Sea of Chiloé (ISC) in northwestern Patagonia has experienced large harmful algal blooms in the past decade, impacting human health and affecting the large aquaculture industry of the region. Thus, the investigation of factors favouring regional phytoplankton growth are of particular interest. Analysing the synoptic-to-intraseasonal variability, we explore changes in phytoplankton biomass in southern ISC (S-ISC, 42.5°-43.5°S, 72.5°-74°W) and their concurrent mesoscale and large-scale meteorological and oceanographic conditions. We use high-resolution satellite normalized fluorescence line height (nFLH) and chlorophyll-a (CHL-A) from the MODIS-Aqua sensor as proxies for phytoplankton biomass, besides oceanic and atmospheric variables derived from various remote-sensing sources and atmospheric fields from the ERA5 reanalysis. Specifically, we focus on high phytoplankton biomass events HBEs, which are defined as those cases when intraseasonal nFLH anomaly (nFLH’) exceeds the 95th percentile threshold. Each event was characterised by its first date of occurrence (called day 0). We detected 16 HBE between 2003 and 2019 in S-ISC. HBEs tend to occur under the influence of a mid-latitude migratory anticyclone that induce persistent cloudless conditions preceding day 0, leading to enhanced photosynthetically active radiation (PAR) starting around day -8, and positive sea surface temperature (SST) anomalies between days -4 and +4. We hypothesise that HBEs are mainly modulated by i) mixing and advection that could contribute to a greater availability of nutrients in the upper sea layers before the onset of the anticyclonic anomalies; and ii) increased thermal stratification related to positive PAR and SST anomalies that would promote phytoplankton growth during the anticyclonic regime. Furthermore, we show that the Madden-Julian Oscillation modulates the frequency of nFLH’ and thus of HBEs, a result that suggests an enhanced predictability of these cases.

Dynamics of phytoplankton functional groups in river-connected lakes and the major influencing factors: A case study of Dongting Lake, China

Identifying the drivers that impact phytoplankton biomass and dynamics of functional groups (FGs) is crucial to the management of lake aquatic systems. To explore the interactive effects of hydrological characteristics, physicochemical parameters and phytoplankton FGs in the largest river-connected lake, a case study was carried out at Dongting Lake, China. We analyzed the changes in the hydrological characteristics of the lake as well as in the phytoplankton biomass throughout the year to explore the driving factors that influence phytoplankton FGs. Four periods were defined in our study based on water level (WL) fluctuations including the rising periods (RⅠ and RⅡ) and the falling periods (FⅠ and FⅡ). The phytoplankton biomass in Dongting Lake was the highest in RⅡ, followed by RⅠ, FⅠ, and FⅡ. And the trend of FGs dominance through the four periods were P/MP/D (RⅠ) → P/MP/J (RⅡ) → MP (FⅠ) → MP/P/Y (FⅡ). Changes in phytoplankton biomass and FGs were the result of a combination of hydrological characteristics and physicochemical parameters. Throughout the water period, the hydrological characteristics did not directly influence phytoplankton, instead exerting a more indirect effect on phytoplankton biomass and FGs by affecting the water’s physicochemical parameters, e.g., permanganate index (CODMn), nitrogen nutrients, dissolved oxygen (DO), water temperature (WT), and conductivity (Cond). CODMn might be a significant predictor of phytoplankton biomass and FGs in all water periods, explaining 36.6% of the variability, followed by Cond, WT, ammonia nitrogen (NH4+-N), and DO explaining 17.6%, 12.3%, 11.3%, and 7.2%, respectively. However, according to each water phase analysis, the WL was the key factor for phytoplankton growth in RⅡ. Nitrogen nutrients, WT and Cond had more pronounced effects on phytoplankton in RⅠ, FⅠ and FⅡ, respectively., Based on the findings of our study, we suggest that increased control of organic matter input into the lake and nitrogen fertilizer use at nearby agricultural surface sources are effective actions that are needed to prevent and control algal blooms in Dongting Lake.

Effects of Climate Change on Chlorophyll a in the Barents Sea: A Long-Term Assessment.

The Arctic climate strongly affects phytoplankton production and biomass through several mechanisms, including warming, sea ice retreat, and global atmospheric processes. In order to detect the climatic changes in phytoplankton biomass, long-term variability of chlorophyll a (Chl-a) was estimated in situ with the changes in the surface sea temperature (SST) and salinity (SSS) in the Barents Sea and adjacent waters during the period of 1984-2021. Spatial differences were detected in SST, SSS, and Chl-a. Chl-a increased parallel to SST in the summer-autumn and spring periods, respectively. Chl-a peaks were found near the ice edge and frontal zones in the spring season, while the highest measures were observed in the coastal regions during the summer seasons. SST and Chl-a demonstrated increasing trends with greater values during 2010-2020. Generalized additive models (GAMs) revealed that SST and Chl-a were positively related with year. Climatic and oceanographic variables explained significant proportions of the Chl-a fluctuations, with six predictors (SST, annual North Atlantic Oscillation index, temperature/salinity anomalies at the Kola Section, and sea ice extent in April and September) being the most important. GAMs showed close associations between increasing Chl-a and a decline in sea ice extent and rising water temperature. Our data may be useful for monitoring the Arctic regions during the era of global changes and provide a basis for future research on factors driving phytoplankton assemblages and primary productivity in the Barents Sea.

Application of multidimensional analysis methods in studying the dynamics of planktonic cyanobacteria, diatoms and chlorophyta biomass in the southern part of Lake Peipus

The influence of hydrological and hydrochemical conditions on the long-term biomass dynamics is studied using three taxonomic groups of microalgae in Lake Peipus (Cyanobacteria, Chlorophyta and Diatoma). Cluster analysis based on long-term phytoplankton biomass dynamics in the lake showed that blue-green algae stood apart, while diatoms and green algae were quite similar to each other. The rationale for the application of principal components analysis is that this procedure allows reducing the data set and creating independent components, making it much easier to describe patterns in the systems. Before the analysis, data were checked for multicollinearity using correlation analysis. Analysis revealed five principal components which determined the patterns of change in phytoplankton biomass. It is shown that Cyanobacteria depend the most on the lake temperature regime and the content of dissolved mineral and organic compounds in the water, which is in full agreement with the data reported by other authors. Chlorophyta, compared with blue-green algae, are more sensitive to the acidbase balance of the water. As for diatoms, only the water level and the duration of the ice-covered period matter

Термохалинные характеристики и пространственное распределение хлорофилла на стандартном разрезе “Кольский меридиан” в апреле 2021 года

In the Barents Sea, the major part of the annual primary pelagic production is formed during the spring phytoplankton bloom. A phytoplankton bloom is understood as an annually recurring increase in its total biomass. Chlorophyll a concentration reflects changes in phytoplankton biomass and can serve as an indicator of production characteristics. In April 2021, the thermohaline characteristics of the water column and the distribution of chlorophyll a concentrations along a standard transect “Kola Section” were studied. The Transect was located in the path of the warm waters of the Nordkapp current and the desalinated waters of the Murmansk coastal current. Coastal, Atlantic and Arctic waters were defined along the Transect. Chlorophyll a concentrations in coastal waters varied on average from 0.57 to 0.86 mg/m3 (maximum 1.11 mg/m3), in Atlantic waters from 0.11 to 0.34 mg/m3 (maximum 0.50 mg/m3). At the station located in the area of intensive interaction of two types of waters (in the area of salinity and temperature fronts) maximum concentrations of chlorophyll a (0.96–1.39 mg/m3) were noted. The location and extent of zones of increased (or decreased) chlorophyll a concentrations agreed with the alternation of water masses. The beginning of the early spring phytoplankton bloom was in the coastal zone of Central Murman.

Phytoplankton size distributions in the western North Atlantic and their seasonal variability

AbstractPhytoplankton play a major role on Earth, impacting the global distribution and cycles of carbon, oxygen, nitrogen, sulfur, and other elements, and structuring marine food webs. One fundamental trait of phytoplankton with direct biogeochemical implications is their size, as it governs metabolic and sinking rates as well as prey–predator interactions. Phytoplankton size spans approximately 3.5 orders of magnitude (when expressed as an equivalent spherical diameter), and thus measuring the full range in size distribution of phytoplankton is challenging and rarely attempted. Here, we constructed phytoplankton size spectra by merging state‐of‐the‐art cytometry and imaging cytometry measurements that were collected in the western North Atlantic Ocean, along a latitudinal gradient (36°N to 55°N) and during different phases of the annual cycle of phytoplankton. The derived spectra show a seasonal pattern that parallels changes in phytoplankton biomass, and do not always follow a commonly assumed power‐law model. Shifts in size spectra were more pronounced in the sub‐Arctic and temperate subregions, compared to the subtropical region of the study area. We evaluated the relationships between different size groups and environmental parameters to derive ecologically meaningful size groups. Finally, to simulate Ocean Color remote‐sensing algorithms of phytoplankton size, we compared temporal variations in descriptors of the size spectra (median particle size, phytoplankton size distribution exponent) with optical size proxies derived from light absorption and attenuation; good agreement was observed in the northern sections of the study area where temporal changes in community size structure were more pronounced.

Temporal changes of phytoplankton biomass in the western Black Sea shelf waters: Evaluation by satellite data (1998–2018)

This study investigated temporal changes of phytoplankton biomass with particular focus on blooms and their underlying mechanisms within different shelf regions in the western part of the Black Sea for the poorly studied post-eutrophication period. We combined satellite chlorophyll with carbon-to-Chl a ratios to obtain a better proxy of phytoplankton biomass, and applied an algorithm to partition these data into a base seasonal variation and blooms. The base phytoplankton biomass was characterized by a seasonal maximum in October–November, highest values in the Romanian inner shelf region that receives inputs directly from the Danube River, and downward time trends in all shelf regions, coinciding with reduced nitrate inputs from the Danube. The majority of phytoplankton blooms were short-lived events (lasting up to 5 days), which accounted for over 60% of total bloom events. The initiation of spring blooms coincided with the peak riverine input for the Romanian inner shelf and with the formation of the thermocline in other regions. Autumn blooms were observed with the breakdown of the thermocline in October–November. These were most likely sustained by nutrient accumulation in bottom waters originating from riverine inputs in spring and regenerated nutrients from sedimenting organic matter and sediments. Large diatoms may contribute to autumn blooms in the outer shelf regions through harvesting “new” nitrate from deeper waters and hosting endosymbiotic cyanobacteria to extract “new” nitrogen from the atmosphere. Finally, high intensity of upwelling processes could support winter blooms in the Bulgarian outer shelf region.

Effects of Mixed Layer Depth on Phytoplankton Biomass in a Tropical Marginal Ocean: A Multiple Timescale Analysis

AbstractIn open oceans, changes in mixed layer depth (MLD) may affect phytoplankton growth and biomass variations via the regulation of nutrient supply from deep waters. Estimates of relationships between variability in phytoplankton dynamics and the MLD remain limited, especially at different time scales. We compiled and analyzed averaged euphotic‐depth‐integrated chlorophyll‐a (IChl‐a) and surface chlorophyll‐a (SChl‐a) concentrations collected from 27 cruises during the period of 1999–2019 in the tropical northern South China Sea (SCS). Seasonal differences existed in both averaged IChl‐a and SChl‐a concentrations, with significantly high concentrations in the cold season. Inconsistent relationships between the averaged IChl‐a and SChl‐a concentrations between seasons implied that the use of SChl‐a concentration as a common indicator of phytoplankton biomass dynamics should be performed with caution. Over the past decades in the northern SCS, the averaged IChl‐a, SChl‐a, and MLD decreased to a greater extent in the cold season than in the warm season, while sea surface temperature (SST) rose rapidly and dramatically in both seasons. The MLD was observed to have better correlations with the averaged IChl‐a and SChl‐a concentrations than the SST in the time‐series data. Our results highlight the importance of IChl‐a concentration, which is an overall measure of phytoplankton responses to euphotic zone conditions, and the MLD could be used as a good index for changes in phytoplankton biomass under climate change.

Estimating phytoplankton stoichiometry from routinely collected monitoring data.

Accurately estimating the elemental stoichiometry of phytoplankton is critical for understanding biogeochemical cycles. In laboratory experiments, stoichiometric ratios vary among species and with changes in environmental conditions. Field observations of total phosphorus (P) and total nitrogen (N) collected at regional and national scales can supplement and expand insights into factors influencing phytoplankton stoichiometry, but analyses applied to these data can introduce biases that affect interpretations of the observed patterns. We introduce an analytical approach for estimating the ratio between phytoplankton N and P from the particulate fraction of nutrient pools in lake samples. We use Bayesian models to represent observations of particulate P and N as the sum of contributions from nutrients bound within phytoplankton and nutrients associated with non-phytoplankton suspended sediment. Application of this approach to particulate nutrient data collected in Missouri impoundments yields estimates of the mass ratio of N:P in phytoplankton ranging from 8-10 across a variety of lakes and seasons. N:P in particulate matter ranged from 6 to 70, a variability driven by differences in nutrients bound to non-phytoplankton suspended sediment. We adapted the Bayesian models to estimate N:P using more commonly available measurements of total P and total N and applied this model to a continental-scale monitoring data set. We compared phytoplankton nutrient content estimated from the two analyses and found that when datasets lack direct measurements of particulate nutrient concentrations, the model estimate of phytoplankton nutrient content includes contributions from nutrients within phytoplankton and dissolved nutrients that are associated with changes in phytoplankton biomass.

Uncoupled changes in phytoplankton biomass and size structure in the western tropical Atlantic

Structural changes in phytoplankton communities have large influence on marine elemental cycling, food web dynamics and carbon export. Here we used data from two field expeditions, performed in spring and fall, over a coast-offshore gradient to investigate phytoplankton structure and dynamics in the Southwestern Tropical Atlantic (SWTA). Results revealed a predominant role of the thermohaline structure as the main driver of phytoplankton dynamics regardless the season. In fall, the thermocline and nutricline shallowing promoted a biomass increase, which was 3-fold higher around the oceanic islands. The structure of phytoplankton community mainly varied vertically, with Prochlorococcus pigments contributing greatly to the higher nutrient Deep Chlorophyll Maximum, whereas other Cyanophyceae predominated in nutrient poor surface layers during the two seasons. In addition, a clear coast-offshore variability in the new production ( Fp ) was observed, with the shelf region displaying higher values (up to 0.21), promoted by larger Bacillariophyceae pigments concentration, thus suggesting a coastal influence on shelf production. Although the phytoplankton biomass increased seasonally, our results highlighted a predominance of recycled production ( Fp ) and uncoupled dynamics between biomass and size of phytoplankton structure, with pico- and nanophytoplankton dominating the relative biomass, i.e., ca. 80% of the community in both seasons. We hypothesize that these patterns may result from a strong nitrogen limitation (N:P of around 3:1), which likely constrain a pronounced growth of the microphytoplankton. • Uncoupled dynamics between phytoplankton size structure and biomass were revealed. • A decreasing gradient of biomass and new production from shelf to offshore was pointed out. • Seasonal changes in thermocline shape the phytoplankton dynamics and size structure. • Phytoplankton diversity increased with depth, but was mainly dominated by small cells.

Stable isotopes demonstrate seasonally stable benthic-pelagic coupling as newly fixed nutrients are rapidly transferred through food chains in an estuarine fish community.

Seasonal differences in the availability of resources potentially result in the food web architecture also varying through time. Stable isotope analyses are a logistically simple but powerful tool for inferring trophic interactions and food web structure, but relatively few studies quantify seasonal variations in the food web structure or nutrient flux across multiple trophic levels. We determined the temporal dynamics in stable isotope compositions (carbon, nitrogen and sulphur) of a fish community from a highly seasonal, temperate estuary sampled monthly over a full annual cycle. Sulphur isotope values in fish tissues discriminated among consumers exploiting pelagic and benthic resources but showed no seasonal variation. This implied limited change in the relative consumption of pelagic and benthic resources by the fish community over the study period despite major seasonal changes in phytoplankton biomass. Conversely, carbon and nitrogen isotope values exhibited seasonality marked by the commencement of the spring phytoplankton bloom and peak chlorophyll concentration, with δ13 C values following expected trends in phytoplankton growth physiology and variation in δ15 N values coinciding with changes in major nitrogen sources to plankton between nitrate and ammonium. Isotope shifts in fish muscle were detected within 2 weeks of the peak spring phytoplankton bloom, suggesting a rapid trophic transfer of carbon and nitrogen along food chains within the estuarine food web during periods of high production. Therefore we caution against the assumption that temporal averaging effectively dampens isotopic variability in tissues of higher trophic-level animals in highly dynamic ecosystems, such as temperate estuaries. This work highlights how stable isotope analyses can be combined with environmental data to gain a broader understanding of ecosystem functioning, while emphasising the need for temporally appropriate sampling in stable isotope-based studies.

Multi-decadal changes in phytoplankton biomass in northern temperate lakes as seen through the prism of landscape properties.

Ecologists collectively predict that climate change will enhance phytoplankton biomass in northern lakes. Yet there are unique variations in the structures and regulating functions of lakes to make this prediction challengeable and, perhaps, inaccurate. We used archived Landsat TM/ETM+satellite products to estimate epilimnetic chlorophyll-a (Chl-a) concentration as a proxy for phytoplankton biomass in 281 northern temperate lakes over 28years. We explored the influence of climate (air temperature, precipitation) and landscape proxies for nutrient sources (proportion of wetlands in a contributing catchment, size of the littoral zone, potential for wind-driven sediment resuspension as estimated by the dynamic ratio) or nutrient sinks (lake volume) in a random forest model to explain heterogeneity in peak Chl-a. Lakes with higher Chl-a (median Chl-a=2.4μgL-1 , n=40) had smaller volumes (<44×104 m3 ) and were more sensitive to increases in temperature. In contrast, lakes with lower Chl-a (median Chl-a=0.6μgL-1 , n=241) had larger volumes (≥44×104 m3 ), contributing catchments with smaller proportions of wetlands (<4.5% of catchment area, n=70), smaller littoral zones (<16.4ha, n=137), minimal wind-driven sediment resuspension (as defined by the dynamic ratio; <0.45, n=232), and were more sensitive to increases in precipitation. Lakes with larger volumes were generally less responsive to climate factors; however, larger volume lakes with a significant proportion of wetlands and larger littoral zones behaved similarly to lakes with smaller volumes. Our finding that lakes with different landscape properties respond differently to climate factors may help predict the susceptibility of lakes to eutrophication under changing climatic conditions.

Amino acid nitrogen stable isotopes as biomarkers of coastal phytoplankton assemblages and food web interactions

Marine phytoplankton and zooplankton face rapidly changing environments in the face of global warming and climate change. We investigated the effect of warmer water and lower pH conditions—projected for New Zealand coastal waters at the start of the next century—on both phytoplankton and zooplankton in a 20 d mesocosm experiment to determine whether amino acid stable isotopes could be used as biomarkers of environmental change. We also assessed whether key environmental drivers, such as those linked to climate change, altered the processing of amino acids at the base of the food web. Despite changes in phytoplankton biomass and community composition, we found no significant difference in either particulate organic matter (POM) bulk or amino acid-specific δ15N values, indicating that the trophic status of POM was not significantly influenced by lower pH and warming. Threonine δ15N values were the most sensitive to changes in the phytoplankton community and showed correlations with diatoms (positive) and small flagellates (negative), demonstrating potential as a biomarker for detecting changes related to these phytoplankton groups and thus making threonine a promising indirect indicator of climate change. Finally, δ15NPhe values tracked changes in the lower food web, likely due to faster turnover times, showing its valuable role as a tracer of the nitrogen baseline, even during accelerated metabolism in zooplankton.

Do Cross-Latitude and Local Studies Give Similar Predictions of Phytoplankton Responses to Warming? An Analysis of Monitoring Data from 504 Danish Lakes

Cross-latitude studies on lakes have a potential to predict how global warming may cause major changes in phytoplankton biomass and composition, e.g., the development of favourable conditions for cyanobacteria dominance. However, results from these studies may be influenced by biogeographical factors, and the conclusions may, therefore, not hold when considering local response patterns. We used monthly monitoring data from 504 lakes in Denmark—a small and homogeneous geographical region—to establish empirical relationships between key phytoplankton groups and a set of explanatory variables including total phosphorus (TP), total nitrogen (TN), lake mean depth (DEP) and water temperature (TEMP). All variables had strong effects on phytoplankton biomass and composition, but their contributions varied over the seasons, with TEMP being particularly important in June–October. We found dominance of cyanobacteria in terms of biomass and also an increase in dinophytes biomass at higher TEMP, while diatoms and chlorophytes became less important. In May, however, the TEMP effect on total phytoplankton biomass was negative, likely reflecting intensified zooplankton grazing. Our results suggest that biogeographical effects are of minor importance for the response patterns of phytoplankton to temperature and that substantial concentration reductions of TN and TP are needed in eutrophic lakes to counteract the effect of the climate change-induced increase in TEMP.

Response of Bacterial Communities to Rainwater Additions in the Oligotrophic SE Mediterranean Coast; Experimental and In‐Situ Observations

AbstractWet deposition may serve as a source of external nutrients for phytoplankton and heterotrophic bacteria in low nutrients low chlorophyll (LNLC) marine systems. Here, we examined the effects of rainwater deposition on phytoplankton and bacterial abundance and activity in the LNLC SE Mediterranean coast. In‐situ observations and experimentally controlled incubations show enhanced abundance and productivity of the bacterioplankton communities attributed to the addition of rainwater‐born nutrients. While rainwater relieved N limitation to phytoplankton resulting in enhanced abundance and primary production of autotrophic microbial communities (mostly pico‐eukaryotes), no single nutrient (N or P) could be identified as a sole limiting factor of the stimulated heterotrophic bacteria. In both the experimental and in‐situ observations, the changes in phytoplankton biomass following rainwater deposition/addition was observed only after 24–48 h. Contrary, an increase in primary and bacterial productions was visible within a few hours, suggesting high nutrients turnover rates by bacterioplankton communities following rain deposition. These results emphasize the importance of short‐term wet deposition events to new N production and phytoplankton temporal variability in the LNLC SE Mediterranean coast.

Spring Accumulation Rates in North Atlantic Phytoplankton Communities Linked to Alterations in the Balance Between Division and Loss.

For nearly a century, phytoplankton spring blooms have largely been explained in the context of abiotic factors regulating cellular division rates (e.g., mixed-layer light levels). However, the accumulation of new phytoplankton biomass represents a mismatch between phytoplankton division and mortality rates. The balance between division and loss, therefore, has important implications for marine food webs and biogeochemical cycles. A large fraction of phytoplankton mortality is due to the combination of microzooplankton grazing and viral lysis, however, broad scale simultaneous measurements of these mortality processes are scarce. We applied the modified dilution assay along a West-to-East diagonal transect in the North Atlantic during spring. Our results demonstrate positive accumulation rates with losses dominated by microzooplankton grazing. Considering the dynamic light environment phytoplankton experience in the mixed surface layer, particularly in the spring, we tested the potential for incubation light conditions to affect observed rates. Incubations acted as short-term ‘light’ perturbations experiments, in which deeply mixed communities are exposed to elevated light levels. These “light perturbations” increased phytoplankton division rates and resulted in proportional changes in phytoplankton biomass while having no significant effect on mortality rates. These results provide experimental evidence for the Disturbance-Recovery Hypothesis, supporting the tenet that biomass accumulation rates co-vary with the specific rate of change in division.