Timing Of Spring Bloom Research Articles

Land use change and coastal water darkening drive synchronous dynamics in phytoplankton and fish phenology on centennial timescales

AbstractAt high latitudes, the suitable window for timing reproductive events is particularly narrow, promoting tight synchrony between trophic levels. Climate change may disrupt this synchrony due to diverging responses to temperature between, for example, the early life stages of higher trophic levels and their food resources. Evidence for this is equivocal, and the role of compensatory mechanisms is poorly understood. Here, we show how a combination of ocean warming and coastal water darkening drive long‐term changes in phytoplankton spring bloom timing in Lofoten Norway, and how spawning time of Northeast Arctic cod responds in synchrony. Spring bloom timing was derived from hydrographical observations dating back to 1936, while cod spawning time was estimated from weekly fisheries catch and roe landing data since 1877. Our results suggest that land use change and freshwater run‐off causing coastal water darkening has gradually delayed the spring bloom up to the late 1980s after which ocean warming has caused it to advance. The cod appear to track phytoplankton dynamics by timing gonadal development and spawning to maximize overlap between offspring hatch date and predicted resource availability. This finding emphasises the importance of land–ocean coupling for coastal ecosystem functioning, and the potential for fish to adapt through phenotypic plasticity.

Optimal phenology of life history events in Calanus finmarchicus: exit from diapause in relation to interannual variation in spring bloom timing and predation.

Respiration of lipids by copepods during diapause (overwintering dormancy) contributes to ocean carbon sequestration via the seasonal lipid pump (SLP). Parameterizing this flux in predictive models requires a mechanistic understanding of how life history adaptation in copepods shapes their timing of exit from diapause. We investigate the optimal phenology of Calanus finmarchicus in the Norwegian Sea using an individual-based model in which diapause exit is represented as a trait characterized by phenotypic mean and variance. Without interannual variability, optimal exit correlated with the onset of the spring phytoplankton bloom and phenotypic variance was of no benefit. In contrast, copepods endured reduced fitness and adopted bet-hedging strategies when exposed to interannual variability in bloom timing and predation: later exit from diapause and phenotypic variance maintained adult numbers in anomalous late-bloom years. Exit nevertheless remained well before the peak of the bloom which is a favorable strategy when low predation early in the year enhances survival of eggs and early developmental stages. Our work highlights the complex interactions between C. finmarchicus and its environment and the need for improved understanding of bet-hedging strategies and the cues of diapause exit to progress the representation of the SLP in global biogeochemical models.

Spring phytoplankton bloom phenology during recent climate warming on the Bering Sea shelf

High-latitude ecosystems commonly experience large phytoplankton blooms in spring, which provide basal resources for a range of grazers including zooplankton, benthic consumers and fishes. Variation of the timing and intensity of the spring phytoplankton bloom influences the degree of spatial and temporal overlap with consuming organisms. In the Bering Sea, blooms occur in association with ice retreat or as pelagic open-water blooms. In the last few years, the Bering Sea shelf experienced unprecedented and widespread warming. Understanding how those climatic changes subsequently influenced phytoplankton bloom dynamics is critical for evaluating Bering Sea food web responses. We estimate spring bloom timing and type (ice-associated, open-water) across the Bering Sea shelf using a combination of data from the long-running oceanographic moorings on the eastern shelf (M2, M4, M5, M8) and satellite ocean color data from 1998 to 2022. We assess 1) if the Bering Sea shelf experienced noticeable changes in spring bloom timing or type in the last two decades, 2) whether bloom phenology was accentuated by the recent warm period (2018–2019), and 3) what influences do winds and sea surface temperatures have on spring bloom timing and where are these variables influential? Our spatial analyses reinforce the conclusion that ice retreat is the dominant forcing factor of bloom timing for the Bering Sea shelf with some influence of wind for open-water blooms. Overall, bloom timing has not shifted seasonally with climate warming in the last two decades for most of the Bering Sea shelf, except for nearshore areas and mainly in the northern Bering Sea. In warm years when ice retreats early prior to the last week of March, blooms form in open waters and bloom timing on the middle and outer shelf is delayed when wind mixing is prevalent in ice-free springs. In recent years, open-water blooms were more widespread than previously experienced, and even occurred in the northern Bering Sea during 2018–2019. A progression to more open water blooms in a future warmer climate will influence the availability of basal resources for pelagic and benthic consumers.

Asymmetry in the rate of warming and the phenology of seasonal blooms in the Northeast US Shelf Ecosystem

AbstractPredicting the impact of marine ecosystem warming on the timing and magnitude of phytoplankton production is challenging. For example, warming can advance the progression of stratification thereby changing the availability of nutrients to surface phytoplankton, or influence the surface mixed layer depth, thus affecting light availability. Here, we use a time series of sea surface temperature (SST) and chlorophyll remote sensing products to characterize the response of the phytoplankton community to increased temperature in the Northeast US Shelf Ecosystem. The rate of change in SST was higher in the summer than in winter in all ecoregions resulting in little change in the timing and magnitude of the spring thermal transition compared to a significant change in the autumn transition. Along with little phenological shift in spring thermal conditions, there was also no evidence of a change in spring bloom timing and duration. However, we observed a change in autumn bloom timing in the Georges Bank ecoregion, where bloom initiation has shifted from late September to late October between 1998 and 2020—on average 33 d later. Bloom duration in this ecoregion also shortened from ∼7.5 to 5 weeks. The shortened autumn bloom may be caused by later overturn in stratification known to initiate autumn blooms in the region, whereas the timing of light limitation at the end of the bloom remains unchanged. These changes in bloom timing and duration appear to be related to the change in autumn thermal conditions and the significant shift in autumn thermal transition. These results suggest that the spring bloom phenology in this temperate continental shelf ecosystem may be more resilient to thermal climate change effects than blooms occurring in other times of the year.

Evaluating factors affecting the distribution and timing of Pacific Herring Clupea pallasii spawn in British Columbia

Pacific herring Clupea pallasii spawn in nearshore areas in late winter to early spring, but factors influencing the timing and spatial distribution of spawning are not well known. We modeled the temporal and spatial distribution of spawning for 5 herring stocks in British Columbia from egg deposition surveys conducted from 1988-2018 using different sets of environmental predictors and modeling methods. Random forest modeling showed that the timing of spawning in each year was mainly influenced by the number of daylight hours being >10.5, cumulative degree days >100 and salinity at 30.5. The spatial distribution of spawning tended to occur at consistent locations over time. Results showed that the probability of spawning occurring at a transect in a given year was largely determined by the biomass of herring and location of the transect relative to the center of spawning. Environmental factors at individual transects played a much smaller role in determining spawn distribution. There was mixed evidence for spatial expansion of spawning distribution in years of high biomass, with some stocks, such as Haida Gwaii, showing no expansion of the spawning area in years of higher biomass. Since Pacific herring recruitment has been linked to their ability to time larval hatch to spring bloom timing, future warming temperatures may result in earlier herring spawning relative to the spring bloom. This will increase the probability of mismatch with prey, impacting larval herring starvation, growth and perhaps mortality, leading to reductions in recruitment to these important stocks.

Differential response of distinct copepod life history types to spring environmental forcing in Rivers Inlet, British Columbia, Canada.

The temporal dynamics of five copepod species common to coastal waters of the Pacific Northwest were examined in relation to variability in spring temperature and phytoplankton dynamics in 2008, 2009, and 2010 in Rivers Inlet, British Columbia, Canada. The five species were differentiated by life history strategies. Acartia longiremis, Metridia pacifica, and Paraeuchaeta elongata remained active over most of the year. By contrast, the reproductive effort of Eucalanus bungii and Calanus marshallae was concentrated over the spring period and they spent most of the year in diapause as C5 copepodites. A delay in the timing of the spring bloom was associated with a shift in the phenology of all species. However, following the delay in spring bloom timing, recruitment to the G1 cohort was reduced only for E. bungii and C. marshallae. Recruitment successes of E. bungii and C. marshallae was also drastically reduced in 2010, an El Niño year, when spring temperatures were highest. Reasons for the observed differential response to spring environmental forcing, and its effect on upper trophic levels, are discussed.

Analysis of 23 Years of Daily Cloud-Free Chlorophyll and Suspended Particulate Matter in the Greater North Sea

Satellite-derived estimates of ocean color variables are available for several decades now and allow performing studies of the long-term changes occurred in an ecosystem. A daily, gap-free analysis of chlorophyll (CHL) and suspended particulate matter (SPM, indicative of light availability in the subsurface) at 1 km resolution over the Greater North Sea during the period 1998–2020 is presented. Interannual changes are described, with maximum average CHL values increasing during the period 1998–2008, a slightly decreasing trend in 2009–2017 and an stagnation in recent years. The typical spring bloom is observed to happen earlier each year, with about 1 month difference between 1998 and 2020. The duration of the bloom (time between onset and offset) appears also to be increasing with time, but the average CHL value during the spring bloom does not show a clear trend. The causes for earlier spring blooms are still unclear, although a rising water temperature can partially explain them through enhanced phytoplankton cell division rates or through increased water column stratification. SPM values during winter months (prior to the development of the spring bloom) do not exhibit a clear trend over the same period, although slightly higher SPM values are observed in recent years. The influence of sea surface temperature in the spring bloom timing appears to be dominant over the influence of SPM concentration, according to our results. The number of satellites available over the years for producing CHL and SPM in this work has an influence in the total amount of available data before interpolation. The amount of missing data has an influence in the total variability that is retained in the final dataset, and our results suggest that at least three satellites would be needed for a good representation of ocean color variability.

Effects of sea ice and wind speed on phytoplankton spring bloom in central and southern Baltic Sea.

In this study, the effects of sea ice and wind speed on the timing and composition of phytoplankton spring bloom in the central and southern Baltic Sea are investigated by a hydrodynamic-biogeochemical model and observational data. The modelling experiment compared the results of a reference run in the presence of sea ice with those of a run in the absence of sea ice, which confirmed that ecological conditions differed significantly for both the scenarios. It has been found that diatoms dominate the phytoplankton biomass in the absence of sea ice, whereas dinoflagellates dominate the biomass in the presence of thin sea ice. The study concludes that under moderate ice conditions (representing the last few decades), dinoflagellates dominate the spring bloom phytoplankton biomass in the Baltic Sea, whereas diatoms will be dominant in the future as a result of climate change i.e. in the absence of sea ice.

Temperature-related timing of the spring bloom and match between phytoplankton and zooplankton

ABSTRACT Global warming is causing changes in the food web structure and seasonal plankton dynamics. The Baltic Sea is one of the fastest-warming sea areas and warming consequently affects the timing and magnitude of phytoplankton blooms. Based on available Chlorophyll a data from nine years between 1979 and 2018, from the entrance to the Gulf of Finland, we studied the timing of the phytoplankton spring bloom in relation to spring seawater temperature. We found the peak of the bloom to occur earlier in years with higher spring seawater temperature. In warmer years, there was also a shorter time lag between phytoplankton and zooplankton biomass peaks. In addition, it seems as the spring bloom total biomass has decreased with time, with more extensive summer blooms during later years, in line with the general trend observed in the Baltic Sea. The spring bloom has traditionally been considered the most important part of the season, but we argue that the whole growth season should be investigated since summer blooms appear to increase with warming.

Eat or Sleep: Availability of Winter Prey Explains Mid-Winter and Spring Activity in an Arctic Calanus Population

Copepods of the genus Calanus have adapted to high levels of seasonality in prey availability by entering a period of hibernation during winter known as diapause, but repeated observations of active Calanus spp. have been made in January in high latitude fjords which suggests plasticity in over-wintering strategies. During the last decade, the period of Polar Night has been studied intensively in the Arctic. A continuous presence of an active microbial food web suggests the prevalence of low-level alternative copepod prey (such as microzooplankton) throughout this period of darkness. Here we provide further evidence of mid-winter zooplankton activity using a decadal record of moored acoustics from Kongsfjorden, Svalbard. We apply an individual based life-history model to investigate the fitness consequences of a range of over-wintering strategies (in terms of diapause timing and duration) under a variety of prey availability scenarios. In scenarios of no winter prey availability (Pwin = 0 g C L􀀀1), the optimal time to exit diapause is in March. However, as Pwin increases (up to 40 g C L􀀀1), there is little fitness difference in copepods exiting diapause in January compared to March. From this, we suggest that Calanus are able (in energetic terms) to either i) exit diapause early to deal with uncertainty in spring bloom timing, or ii) remain active throughout winter if diapause is not possible (i.e. environment not deep enough, or not enough lipid reserves built up over the previous summer). The range of viable overwintering strategies increases with increasing Pwin, suggesting that there is more flexibility for Calanus spp. in a scenario of non-zero Pwin.

Autonomous Multi-Trophic Observations of Productivity and Export at the Australian Southern Ocean Time Series (SOTS) Reveal Sequential Mechanisms of Physical-Biological Coupling

The timing of pelagic spring blooms has received attention to understand controls on open ocean productivity and its potential responses to climate change. Many studies have relied on surface chlorophyll (Chl) to define bloom initiation because of its availability from satellite observations, but this has limited utility because it ignores the full water column budget and because biomass represents only the small residual term in the balance between production and loss. Additional important measures include net community production (NCP) which determines maximal energy available to fuel phytoplankton and higher trophic level biomass accumulations, and particulate organic carbon export (POC flux) which determines the distribution of this energy across pelagic, mesopelagic and benthic communities. Here we present high temporal resolution records for the winter to spring transition (July-December 2012) obtained from moored sensors at SOTS in the Subantarctic Zone (SAZ) south of Australia. Measurements included physical drivers (temperature, salinity, surface mixed layer depth, currents, wind speeds, insolation, and air-sea heat fluxes) and biological responses (Chl from fluorescence and light attenuation, NCP from O2/N2 ratios and nutrient concentrations from an autonomous water sampler, POC flux from sediment traps, and zooplankton abundances from four-frequency acoustic backscatter profiles). These observations provide a phenology across the four trophic levels (NPZD) commonly used in ocean biogeochemical models. Chl column inventories began to increase in early winter while mixed layers were still deepening, and were accompanied by increases in net community production. Acoustic metrics for grazing pressure were very low at this time. In contrast, surface Chl did not increase until later when stratification developed. The levels of spring net community production were relatively high and balanced by sinking particle fluxes close to global median values, despite the relatively low surface biomass levels. Overall this phenology suggests that the extent of exchange with SAMW waters via deep mixing is a key driver of the seasonality of production, support of higher trophic levels, and the mediation of pelagic-benthic coupling, and occurs sequentially via trophodynamic (de-coupling of production and grazing) and physical (stratification) mechanisms.

Century-scale changes in phytoplankton phenology in the Gulf of Maine.

The phenology of major seasonal events is an important indicator of climate. We analyzed multiple datasets of in situ chlorophyll measurements from the Gulf of Maine dating back to the early 20th century in order to detect climate-scale changes in phenology. The seasonal cycle was consistently characterized by a two-bloom pattern, with spring and autumn blooms. The timing of both spring and autumn blooms has shifted later in the year at rates ranging from ∼1 to 9 days per decade since 1960, depending on the phenology metric, and trends only emerged at time scales of >40 years. Bloom phenology had only weak correlations with major climate indices. There were stronger associations between bloom timing and physical and chemical variables. Autumn bloom initiation correlated strongly with surface temperature and salinity, and spring bloom with nutrients. A later spring bloom also correlated with an increased cohort of Calanus finmarchicus, suggesting broader ecosystem implications of phytoplankton phenology.

Regional differences and inter-annual variability in the timing of surface phytoplankton blooms in the Labrador Sea

The annual phytoplankton bloom is an important marine event and its inter-annual variability can be used to monitor changes in the pelagic ecosystem. In this context, the use of a bio-regionalization analysis to objectively identify ocean areas that differ in their biological characteristics and environmental conditions may be useful to disentangle the functioning of the ecosystem. In this work, satellite surface chlorophyll-a concentration observations were used to obtain a bio-regionalization of the Labrador Sea (LS). Specifically, bioregions were defined using a K-means cluster analysis applied on the satellite time-series of chlorophyll-a concentration. Finally, the resulting biologically regionalization was used in conjunction with simulated data from a state-of-the-art ocean circulation model (NEMO) to investigate difference in spring bloom timing response across the Labrador Sea (LS). Chlorophyll-a seasonal cycle in the LS varies markedly between two neighboring but distinct bioregions: the north (>60 °N) and the south (<60 °N). The north LS blooms earlier (∼early-mid April) and more intensely, while the south LS blooms later (∼May) and its duration may persist all summer long. In the southern LS bioregion, the maximum phytoplankton growth rate (i.e., the initiation of the surface spring bloom) coincides closely with the timing of the first cooling-to-heating shift in air-sea heat fluxes. In this area, the bloom timing tends to precede the shoaling of the mixed-layer depth and the vernal development of the upper ocean stratification. In the southern bioregion, the cooling-to-heating shift in air-sea heat fluxes provided a more suitable indicator for the spring bloom initiation. Meanwhile, in the northern LS bioregion, the early onset of the spring bloom is related to the seasonal evolution (i.e., shoaling) of the mixed-layer depth and precedes the cessation of wintertime cooling. Over the northern bioregion, freshwater exchanges may suppress vertical mixing while the air-sea fluxes are still negative (i.e., ocean cooling). Given the heterogeneity of the LS basin, this study emphasizes the importance to consider the bioregion-specific differences in the upper ocean physical-biological interactions.

Seasonality of the Physical and Biogeochemical Hydrography in the Inflow to the Arctic Ocean Through Fram Strait

Eastern Fram Strait and the shelf slope region north of Svalbard is dominated by the advection of warm, salty and nutrient-rich Atlantic Water (AW). This oceanic heat contributes to keeping the area relatively free of ice. The last years have seen a dramatic decrease in regional sea ice extent, which is expected to drive large increases in pelagic primary production and thereby changes in marine ecology and nutrient cycling. In a concerted effort, we conducted five cruises to the area in winter, spring, summer and fall of 2014, in order to understand the physical and biogeochemical controls of carbon cycling, for the first time from a year-round point of view. We document (1) the offshore location of the wintertime front between salty AW and fresher Surface Water in the ocean surface, (2) thermal convection of Atlantic Water over the shelf slope, likely enhancing vertical nnutrient fluxes, and (3) the importance of ice melt derived upper ocean stratification for the spring bloom timing. Our findings strongly confirm the hypothesis that this ``Atlantification'', as it has been called, of the shelf slope area north of Svalbard resulting from the advection of AW alleviates both nutrient and light limitations at the same time, leading to increased pelagic primary productivity in this region.

Seasonal zooplankton development in a temperate semi-enclosed basin: two years with different spring bloom timing

Abstract Coastal temperate waters undergo considerable intra- and interannual environmental variations, which is reflected in the dynamic nature of their zooplankton communities. Since zooplankton phenology is dependent on several factors, particularly temperature and spring bloom timing, it is imperative to understand how zooplankton communities may shift under future climate conditions with warmer temperatures and more variable spring bloom initiation. To examine zooplankton phenology and response to shifts in bloom timing, we analyzed fortnightly zooplankton and environmental samples collected in the northern Strait of Georgia (B.C., Canada), a large semi-enclosed temperate basin, in 2015 and 2016. Despite a 5-week difference in spring bloom timing, zooplankton community succession was remarkably similar between years. In both years, biomass peaked within the same calendar week and communities were separated into winter, early spring and summer-autumn assemblages that formed independent of the spring bloom timing. Although some species-level phenological differences were observed between years, predominately delayed population development, zooplankton communities appeared to demonstrate resilience to interannual environmental variations on the whole. If ongoing warming shifts the timing of zooplankton consumers’ life history timing, it could lead to a mismatch with their zooplankton prey resource that exhibits comparatively less interannual variability.

Satellite observations of seeding of the spring bloom in the Strait of Georgia, BC, Canada

ABSTRACTWe extend our previous observations of the spring bloom in the Strait of Georgia, Canada using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images, to include both a longer time period (2000–2016) and a wider variety of ‘seeding’ bloom events. We have reported on blooms in inlets to the north of the Strait, which appear to seed the main spring bloom in a characteristic ‘Malaspina Dragon’ pattern, leading to significantly earlier spring bloom dates in some years. Here, we double the time period of those observations. We find no clear repetitions of the earlier dragon pattern, but several instances of seeding from other inlets, with varying impacts on spring bloom timing. In situ monitoring has increased in coverage in recent years in the main body of the Strait, and we have deployed a fluorometer to record blooms at one inlet location, but observations of blooms in inlets rely mostly on satellites. We discuss the spatial and temporal resolution requirements imposed by the spatial scale of inlets and the temporal scale of blooms and cloud cover. In these waters, we find chlorophyll fluorescence images more useful than the standard chlorophyll product, restricting the value of NASA’s Visible Infrared Imaging Radiometer Suite (VIIRS) sensor and suggesting increased value from ESA’s Ocean and Land Colour Imager (OLCI) in the future, especially when two instruments are in orbit.

Use of satellite data to identify critical periods for early life survival of northern shrimp in the Gulf of Maine

AbstractThe northern shrimp Pandalus borealis is at its southern limit in the Gulf of Maine (GOM), and recruitment success is higher in years with relatively cool water temperature. However, the mechanisms for the temperature effect are not clear. We used rolling window analysis of daily satellite data to identify critical periods for early life survival of the 1998–2012 northern shrimp year‐classes and to investigate the importance of the phenology of the hatch and bloom. Survival was negatively correlated with sea surface temperature (SST) during a 6‐week period around the time of larval emergence (late winter) and during a 4‐week period in late summer when SST and stratification reached annual maxima. Survival was negatively correlated with chlorophyll‐a concentration (chl‐a) during two 5‐week periods centered approximately a month before the hatch midpoint and around the time of settlement to the benthos. A small‐magnitude winter bloom occurred around the time of the hatch in many years, but our results did not reveal a link between survival and bloom‐hatch phenology. The timing of winter and spring blooms were correlated with SST during the preceding 10 months. A survival model including SST and chl‐a during the critical periods explained 73% of the variance in survival. Summer SST increased significantly during the study period; the other critical variables showed no trend. The rolling windows approach revealed sensitive periods in early life history that may not have otherwise been hypothesized, providing a foundation for research towards a greater understanding of processes affecting recruitment.

Spring plankton dynamics in the Eastern Bering Sea, 1971–2050: Mechanisms of interannual variability diagnosed with a numerical model

AbstractA new planktonic ecosystem model was constructed for the Eastern Bering Sea based on observations from the 2007–2010 BEST/BSIERP (Bering Ecosystem Study/Bering Sea Integrated Ecosystem Research Program) field program. When run with forcing from a data‐assimilative ice‐ocean hindcast of 1971–2012, the model performs well against observations of spring bloom time evolution (phytoplankton and microzooplankton biomass, growth and grazing rates, and ratios among new, regenerated, and export production). On the southern middle shelf (57°N, station M2), the model replicates the generally inverse relationship between ice‐retreat timing and spring bloom timing known from observations, and the simpler direct relationship between the two that has been observed on the northern middle shelf (62°N, station M8). The relationship between simulated mean primary production and mean temperature in spring (15 February to 15 July) is generally positive, although this was found to be an indirect relationship which does not continue to apply across a future projection of temperature and ice cover in the 2040s. At M2, the leading direct controls on total spring primary production are found to be advective and turbulent nutrient supply, suggesting that mesoscale, wind‐driven processes—advective transport and storminess—may be crucial to long‐term trends in spring primary production in the southeastern Bering Sea, with temperature and ice cover playing only indirect roles. Sensitivity experiments suggest that direct dependence of planktonic growth and metabolic rates on temperature is less significant overall than the other drivers correlated with temperature described above.

Linking phytoplankton phenology to salmon productivity along a north–south gradient in the Northeast Pacific Ocean

We investigated spatial and temporal components of phytoplankton dynamics in the Northeast Pacific Ocean to better understand the mechanisms linking biological oceanographic conditions to productivity of 27 pink salmon (Oncorhynchus gorbuscha) stocks. Specifically, we used spatial covariance functions in combination with multistock spawner–recruit analyses to model relationships among satellite-derived chlorophyll a concentrations, initiation date of the spring phytoplankton bloom, and salmon productivity. For all variables, positive spatial covariation was strongest at the regional scale (0–800 km) with no covariation beyond 1500 km. Spring bloom timing was significantly correlated with salmon productivity for both northern (Alaska) and southern (British Columbia) populations, although the correlations were opposite in sign. An early spring bloom was associated with higher productivity for northern populations and lower productivity for southern populations. Furthermore, the spring bloom initiation date was always a better predictor of salmon productivity than mean chlorophyll a concentration. Our results suggest that changes in spring bloom timing resulting from natural climate variability or anthropogenic climate change could potentially cause latitudinal shifts in salmon productivity.

Contrasting long-term trends and shifts in phytoplankton dynamics in two large rivers

Knowledge about long-term dynamics of phytoplankton in river ecosystems as well as the physical and chemical drivers that potentially control the plankton is essential for predicting future developments, e.g. in response to global climate change. The present study analyzes long-term trends in phytoplankton biomass and shifts in the timing of phytoplankton spring blooms observed in the large rivers Rhine and Elbe from 1990–2009 and 1994–2009, respectively, and analyzes the factors potentially regulating phytoplankton biomass. While phytoplankton biomass in the Elbe was high (seasonal mean chlorophyll-a concentration: 62 µg/L) and showed an increasing tendency, it was much lower in the Rhine (seasonal mean chlorophylla concentration: 10 µg/L) where it decreased significantly during the study period. This decrease coincided with an earlier occurrence of the phytoplankton spring maximum. In the Elbe, the timing of low discharge conditions was crucial for the occurrence of the spring bloom, i.e. an earlier end of the discharge maximum was connected with an earlier spring bloom. In the Rhine, we found a positive correlation between the timing of the spring bloom and the end of winter flood flow. The maximum chlorophylla values during the bloom correlated with the timing of maximum light availability in the Rhine. The findings indicate that climate related factors, like discharge or light conditions, have a high potential to regulate phytoplankton spring bloom dynamics in large rivers. Such dependence could be relevant for predicting phytoplankton development under climate change.