River Nutrient Loads Research Articles

River-reef connectivity in the Meso-American Region

The accumulation and seasonal impact of riverine discharge on coral reefs of the Meso-American Region (MAR) were estimated using a numerical simulation of river runoff dispersion. River-reef connectivity, or source-sink dynamics of terrestrial runoff was further assessed from more than 400 watersheds of the region onto discrete coral reef areas. Using land use for 2003 and 2004 in the MAR, this work builds upon a Regional Ocean Modeling System simulation of the MAR validated by ocean color satellite data, and on the monthly river nutrient and sediment load and discharge provided by the World Resources Institute using the N-SPECT model. Analysis of the variability of simulated runoff transport to the reefs showed that reefs of the Mesoamerican Barrier Reef System (MBRS) were mostly impacted from June to September, following the peak time of river discharge. At that time, the coastal and oceanic circulations contribute quickly to expel the runoff from the MBRS. High runoff concentration waters leaving the eastern coast of Honduras during the months of October to December return to the southwestern MAR in March as they are entrained in a cyclonic gyre. Coral reefs of the MAR are thus impacted twice, first from the coastal side with runoff of local origin and later from the oceanic side with runoff from mixed origin. High probability of connectivity between rivers and remote reefs is established as this study revealed that river runoff from the north shore of Honduras has a wide-spread impact on most of the coral reefs of southern Belize, while watersheds on the Gulf of Honduras are mostly connected to coral reefs in the northern shore of Honduras. Although the level of remote influence (or runoff concentration reaching the reef) is lower than the local, the cumulative effect of numerous remote river-reef connections remains significant.

Modeling nutrient pollution during a first flood event using HSPF software: Iskar River case study, Bulgaria

The nitrogen and phosphorus concentrations in the Iskar River, Bulgaria, during a first flood event have been modeled. The term “ first flood event” refers to the response to the first rainfall event after a long dry period which has a major impact on river nutrient loads. The HSPF model had been calibrated with data for 2 years (2000–2001) and verified using data for a further year for a sub-catchment of the Iskar River (from the source to Iskar reservoir). In order to test the ability of HSPF to describe nutrient pollution during a flood event, the calibrated daily time step model was run for such an event with two time steps—daily and hourly. The results of the hydrologic simulation gave a difference in water balance for the period of less than 5%. Percentage differences between observed and simulated values for the nitrogen load were calculated as 13.1% and 18% for hourly and daily simulations, respectively. Percentage differences between phosphorus measured and simulated loads were larger at 16.6% and 34.4%. The simulations performed here suggest further application of the HSPF model may be valuable in providing a better understanding and in forecasting nutrient concentrations during first flood events.

Human impacts and adaptations in the Caribbean Islands: an historical ecology approach

ABSTRACTArchaeological investigations demonstrate that peoples first settled the Caribbean islands approximately 6000–7000 years ago. At least four major, and multiple minor, migrations took place over the next millennia by peoples from Mesoamerica and South America who practised various subsistence strategies and had different levels of technology. For decades, researchers have been interested in investigating how these groups adapted to and impacted insular environments through time. This paper combines archaeological, palaeoecological, historical, and modern biological data to examine the effects of humans on Caribbean island ecosystems using a historical ecology approach. By synthesising a wide range of data sources, we take a human/nature dialectical perspective to understanding how peoples adapted to and modified their environments. The data suggest that earlier foraging/fishing Archaic groups (ca. 6000–3000 BP), who used a stone tool and shell technology and transported few, if any non-indigenous plants or animals, still impacted island landscapes as evidenced by bird and sloth extinctions. As more advanced ceramic making horticulturalists entered the Antillean chain around 2500 BP, there is an observable change to island environments as a result of forest clearance, overexploitation of both terrestrial and marine resources, and growing populations. Palaeoecological and palaeoenvironmental records also suggest, however, that an increased moisture regime during the late Holocene probably led to a decrease in near-shore salinity and heavier sediment and nutrient loads in rivers. These conditions would have been exacerbated by land clearance for agriculture, leading to coastline progradation, increased turbidity, and mangrove development resulting in changes to the availability of resources for humans on some islands. Although prehistoric peoples in the Caribbean were certainly impacting their environments, it was not until Europeans arrived and population centres grew that intensive and widespread degradation of island landscapes and resources occurred. Modern ecological studies, along with historical and archaeological data, indicate that hundreds of species have been driven to extinction or extirpation – many others have significantly diminished in number, especially within the last two millennia.

Pathways of river nutrients towards the euphotic zone in a deep-reservoir of small size: Uncertainty analysis

The ecological consequences of river inflows, usually carrying large amounts of nutrients, will be different depending on whether it penetrates to the hypolimnion, inserts in the metalimnion or remains on the surface. For plunging rivers, the intrusion depth is controlled by the river-reservoir density difference prior to the plunge point together with mixing processes between ambient and inflowing water, which occur both in the region of the plunge and after the flow has assumed the form of a density current. In contrast with the processes of entrainment into density currents, which have been extensively studied, entrainment and mixing within the plunge zone has received less attention in the physical limnology literature. The existing literature fails to identify adequate parameterizations of this phenomenon, while exhibiting a large variability in the initial mixing rate, measured both in the laboratory and in the field. This work examines the consequences (in terms of uncertainty) that our lack of knowledge of parameters describing river-reservoir mixing in 1-D transport models has on the estimates of river nutrient loads into the euphotic zone of a reservoir, where nutrients are readily available for phytoplankton growth.

The role of aeolian dust in ecosystems

The recent upsurge in research attention to aeolian dust has shown that dust transport systems operate on very large spatial and temporal scales, and involve much larger quantities of sediment than was previously realized. An inevitable consequence of this is that researchers from a range of neighbouring disciplines, including ecology, are beginning to realize that this new knowledge has important implications for their study areas. In the present paper, we examine the ecological implications (real and potential) of this expanding knowledge of dust transport systems, with a particular emphasis upon the Australian dust transport system. We track these ecological effects from source to sink. At source, wind erosion–soil–vegetation relationships are often dominated by temporal changes in rainfall. Nine years of measurements in the Channel Country of the Lake Eyre Basin, Australia show that vegetation and soils in dune fields can recover from drought, whereas on inter-fluve grasslands uni-directional and negative successional vegetation changes can result from wind erosion during drought. On floodplains, both wind erosion and vegetation responses are complicated by flood frequency. Up to 1999 flooding of saline claypans did not increase vegetation but did increase wind erosion through the supply of alluvial fines. However, after three floods within as many months vegetation became established and wind erosion rates were dramatically reduced. Wind erosion research attention is now gradually turning from the physical to the organic content of eroded dusts. In Australia organic matter content can reach 65% by mass, but this cannot be explained by removal of soil organic matter alone. Biological soil crusts not only stabilize soils against wind erosion but contribute to some of the organic dusts. The role of dust as a vector for pathogens is an area which deserves greater research attention in the future. Downwind from source, we show that dust contributions to soils are more widespread and more variable (in time and space) than earlier work on dust-derived loess soils has suggested. Recent studies also show positive dust impacts upon nutrient budgets within distant forest ecosystems, and significant contributions to river nutrient loads, especially in the arid sectors of internally draining river basins. The number of studies of dust impacts upon marine ecosystems is increasing dramatically. Studies of dust within Antarctic ice cores combined with dust modelling provide compelling evidence that increased soluble iron-rich dust inputs to the Southern Ocean have stimulated phytoplankton populations. Modern process studies are, however, yet to clearly demonstrate these relationships. Finally we examine the potential for major dust impacts upon global climates, using its positive and negative effects upon solar radiation and precipitation as examples of the complexity and importance of this new research area.

Diffuse source apportionment of the Po river eutrophying load to the Adriatic sea: Assessment of Lombardy contribution to Po river nutrient load apportionment by means of an integrated modelling approach

The source apportionment of the annual nutrient load carried by the Po river to the Adriatic sea has been studied. An integrated modelling approach was applied to the Lombardy plain area, which covers about 34% of the Po river watershed area and accounts for about 50% of the point sources’ loads carried by the river. To extract all the information available from direct instream measurements, two different modelling tools were alternatively used. The source apportionment was investigated considering both dry and wet weather scenarios. In order to quantify the apportionment in dry-weather conditions, the Lombardy portion of the Po river basin was modelled by using the US-EPA QUAL2E model. Such a simulation allowed to assess a significant contribution (about 50% of the total dry-weather load) of a not rain-driven diffuse pollution component (i.e. groundwater, springs, lake emissaries). Moreover, to estimate the rain-driven surface runoff contribution to the instream total load, the Lombardy plain area was also modelled by means of the US-DA SWAT model. SWAT results indicate a runoff contribution to the Po river instream total load of about 10 000 t N yr −1 and 1300 t P yr −1 (i.e. approximately the 10–20% of the total annual Lombardy nutrient load). At the event scale (i.e. the single rainstorm event) the runoff contribution may rise up to 30–80% of the total instream load. Finally, the total annual nitrogen load at the Po basin closure was estimated for the period 1985–2001. Out of a total annual load of 140 000 t N yr −1, Lombardy accounts for 43% (point plus diffuse sources). The rain-driven diffuse sources constitute the 20% of the overall total load, the point sources account for 40%, whereas the remaining 40% is mainly constituted by “dry-weather diffuse sources” (i.e. groundwater, springs, lake emissaries).

Macroalgal Population and Sustainability

Algal blooms, often associated with eutrophication, are common in temperate regions in coastal lagoons and estuarine areas as well as in open seas. However, because of recent changes in water flows and sediment and nutrient loads in most rivers, eutrophication phenomena are now occurring in areas previously non affected. One of its most widespread and easily recognized effects in a coastal marine ecosystem is the excessive growths and drifts of macroalgae, to such a degree that the biomass becomes a significant problem. The nutrients, notably from anthropogenic origin, constitute a significant factor in the appearance of the blooms in coastal waters. Besides a background originating from natural areas and precipitations, nutrients come from agriculture, sewage, industry, and aquaculture, in very various proportions following the local conditions. At first, the development of opportunist and tolerant seaweeds acts as a purifying system. Then, when the seaweed is stranded or the environmental conditions become unfavourable, the plants die and decompose. The degradation of the chemical equilibrium induces the break-down of the biological balance in the ecosystem. At this stage, a large biomass can become troublesome, resulting in an increase in the herbivore population, competition or toxicity towards flora and fauna, alteration of the sediment, recycling of nutrients and pollutants in the ecosystem, nuisance for local residents and reduction of tourism (beaches degradation, chironomids, odors). Sooner or later, the water deteriorates, which may have a toxic effect on fauna including the commercial fish stocks. Eutrophication problems have thus become a matter of major concern. Its management is neither a simple nor a cheap task. But, environmental and sustainability concerns may, in a not too distant future, override the lack of financial benefit. At any rate, underlayed action is required because in estuarine and potamologic milieus, potential land disposal arrangements are becoming rarer and costlier. Control of eutrophication can only be reached effectively by drastic reduction of the total nutrient load of an overloaded water system. In order to be successful, only an integrated approach, based on a water body's nutrient mass balance and taking into consideration specific geographical, climatological and ecological conditions, can be effective. Moreover, it may take many years before recovery of a eutrophicated ecosystem, because the sediments constitute a complementary source of nutrients for macroalgae. But, prevention will prevent the nutrients from being discharged in the lagoons or in the sea. It will concern the waste treatment, agricultural mode change and treatment of gaseous effluent from polluting human activities.

Exploring changes in river nitrogen export to the world's oceans

Anthropogenic disturbance of river nutrient loads and export to coastal marine systems is a major global problem affecting water quality and biodiversity. Nitrogen is the major nutrient in rivers. On the basis of projections for food production and wastewater effluents, the global river N flux to coastal marine systems is shown to increase by 13% in the coming 3 decades. While the river N flux will grow by about 10% in North America and Oceania and will decrease in Europe, a 27% increase is projected for developing countries, which is a continuation of the trend observed in the past decades. This is a consequence of increasing nitrogen inputs to surface water associated with urbanization, sanitation, development of sewerage systems, and lagging wastewater treatment, as well as increasing food production and associated inputs of N fertilizer, animal manure, atmospheric N deposition, and biological N fixation in agricultural systems. Growing river N loads will lead to increased incidence of problems associated with eutrophication in coastal seas.

River Nutrient Loads and Catchment Size

We have used a total of 496 sample sites to calibrate a simple regression model for calculating dissolved inorganic nutrient fluxes via runoff to the ocean. The regression uses the logarithms of ru ...

Effects of changing nutrient loads to the North Sea

The environmental effects of river nutrient loads to the North Sea have been investigated using a numerical biophysical model, NORWECOM, to perform different reduction scenarios. The simulations demonstrate that the river nutrients have a significant contribution on the annual primary production, both in the southern North Sea, in Skagerrak and along the Norwegian west coast. A 50% reduction in the loads of N and P reduces the primary production with 10–30% in the southern North Sea, and 5–10% in Skagerrak and along the Norwegian west coast. Scandinavian rivers only contribute to the 1–2% level in these reductions, thus, continental rivers have the major effect on the environment in all downstream areas. However, it should be noted that this reduction, even in the southern North Sea, is less than the natural variability of the production of phytoplankton. A reduction only in the P values shows that the production regime in the southern North Sea is phosphorous limited, while nitrogen is the limiting nutrient in the northern North Sea. Focusing on the N/P ratio as a possible proxy for eutrophication, a reduction in the N and P loads reduces this ratio by a similar factor, while a reduction in the P loads only, increases it. Based on this, it is proposed to use the N/P ratio for eutrophication assessment, and this is demonstrated.

Experimental simulations with an ecosystem model of the Baltic Sea: A nutrient load reduction experiment

The study presents results of decadal simulations of the dynamics of the ecosystem of the Baltic Sea with a three‐dimensional coupled physical chemical biological model for the 1980s. The model consists of a circulation model, which is an implementation of the Modular Ocean Model (MOM 2) for the Baltic Sea and an embedded ecosystem model based on a nine‐component model (ERGOM [Neumann, 2000]). The model was driven with realistic atmospheric forcing, river loads, and atmospheric deposition of nutrients. A simulation with the realistic forcing was carried out as a control run in order to study in a further experimental simulation the response of the model ecosystem to a 50% reduction of riverine nutrient loads. It was found that the model food web reacts on the load reduction in a complex manner. The changes in dissolved nutrients and in phytoplankton display a spatial variability and different species‐specific reactions. While the total biomass and nutrient inventories are reduced, there are significant regional differences. In particular, the role of cyanobacteria, which can fix atmospheric nitrogen, is important, and the biomass of cyanobacteria increases in response to the reduced loads. Although the model system needs further improvements, the present study is a step toward an understanding of the quantitative effects of a reduction of nutrient river loads to the Baltic Sea ecosystem based on a full three‐dimensional coupled model.

Effects of river nutrient load reduction on the eutrophication of the North Sea, simulated with the ecosystem model ERSEM

The results of the ecosystem model ERSEM showed, that a reduction in the nutrient load by 50% for N and P cannot be linearly transferred to a similar reduction in primary production in comparison to the standard run for the year 1988. While the reduction scenario results in decreased winter concentrations of nitrogen and phosphorus of up to 40%, the decrease in net primary production reached only up to 20% in small areas in the coastal zone. The phytoplankton groups indicated different reactions to the changed nutrient availability. Generally, there were significant changes in the strength and timing of the nutrient limitation in all phytoplankton groups in the model, but the diatom concentration did not change much. Differences did occur for the flagellates, with sporadically higher flagellate concentration in comparison to the standard run. This result is important, because the increase in algal biomass due to eutrophication was related mainly to an increase in flagellates, which are not decreasing accordingly in the reduction scenario. The reduction scenarios demonstrated that changes in the discharges of the major rivers hardly affect the central North Sea, but lead to significant regional differences in the net primary production. Greatest differences with regard to primary production were found downstream of the river Rhine and Elbe. This leads to changes in the mass flows in the coastal area with an increased importance of the microbial loop. One possible reason for the muted reaction of primary production to decreasing nutrient inputs can be seen in the temporal coincidence of maximum river inputs and the phytoplankton spring bloom. Due to the high nutrient uptake during the spring bloom, inorganic nutrients are bound in the phytoplankton and form a potential for remineralisation. With a more efficient microbial loop, the system becomes less dependent on riverine nutrient inputs in summer.

The monetary benefits of nutrient abatement in the Rhine basin for the water treatment plant of WRK at Andijk

Excessive nutrient loads in rivers and lakes have adverse effects on ecosystem functioning and human activities. The International Rhine Committee has tried to reverse this trend by formulating emission reduction objectives. One of the human activities that may benefit from this is water production that uses surface water as its main source. This paper explores whether reductions in nutrient loads in the Rhine basin result in changes in (the intensity of) treatment activities by the water treatment plant of the Water-transport-company Rhine-Kennemerland (WRK) at Andijk, the Netherlands. For this purpose, a regression analysis was performed on data from 1990–1995 to find a quantitative relationship between the water quality objective for the end product on the one hand, and water quality parameters in Lake IJssel and various treatment processes on the other hand. In the years 1985–1995, phosphorus loads to Lake IJssel reduced by 50% as a result of significant abatement efforts in the Rhine river basin. As a result, chlorophyll-a concentrations decreased by less than 10%. Further phosphorus emission reductions in the Rhine river basin, for example 75% by the year 2000 as is the objective in the Rhine Action Programme, are expected to result in a 30% decrease in chlorophyll-a concentrations compared with the levels of 1985. These reductions in chlorophyll-a concentrations can be expected to have some impacts on the water treatment plant of WRK at Andijk. However, as they constitute less than 1% of production costs, the monetary benefits of these reductions are limited.

Use of fuzzy method to estimate river nutrient loads from scarce observation

Evaluation of data time series in order to get information about water systems is one of the routinely needed tasks. The results are always associated with uncertainties, of which one arises from data scarcity. Traditional methods, such as regression analyses etc. become rapidly useless with decreasing number of data available. A method based on fuzzy set theory was applied to get more reliable information about the system from scarce databases. Monitored daily flow and water quality data of the medium size Zala River in Hungary were considered as elements of fuzzy sets. Fuzzy rules were generated from data pairs (flow, suspended solids concentration, water temperature and phosphorus load as inputs and output, respectively) from which combined rule bases were set up. These rule bases can be considered as a tool of mapping from the input space to the output space using defuzzification procedure. The method is trainable: it can learn from observations. It is demonstrated that the method is capable to generate daily phosphorus loads and annual balance with acceptable accuracy when it is trained only by weekly, biweekly or monthly data pairs. In comparison to other approaches the tool is well suited to utilize better the information content of scarce observations. Furthermore, monitoring costs can be considerably decreased without substantial information loss since sampling of expensive and labour intensive parameters can be reduced.

Modelling nutrient fluxes from source to river load: a macroscopic analysis applied to the Rhine and Elbe basins

In many European rivers, including the major streams of the Rhine and Elbe basins, the nutrient load (N and P) still exceeds target levels. In this paper, a model is presented that describes the river nutrient load as a function of nutrient sources, runoff and lithology in the upstream basin. The model was tested with independent data on nutrient sources (input) and nutrient river load (output) for 130 specific combinations of sub-basins and 5 year periods. A wide range of river systems within the Rhine and Elbe basins were included and the analysis covers a period of 25 years from 1970 to 1995. Most of the observed spatial and temporal variation in the average annual river nutrient load was successfully described by the model. It is therefore concluded that the model can be used to predict the effect of changes in nutrient sources (e.g. reduction of livestock numbers, further improvement of waste water treatment plants etc.) on the average annual nutrient loads of the rivers Rhine, Elbe and their main tributaries.

Analysis of Material Loading to Cannonsville Reservoir: Advantages of Event-Based Sampling

ABSTRACT Eight years of monitoring data are reviewed and analyzed for the West Branch of the Delaware River, the major tributary supplying Cannonsville Reservoir: 3 years are from the early 1980s, the other 5 years include 1991–1996. Phosphorus, nitrogen, and sediment loads were measured at the mouth of the West Branch using an approach of intensified sampling during runoff events and routine sampling during baseflow intervals. Total river loads of nutrients and sediment are reported on a water-year basis; selected analytes are reported on a monthly basis. Additionally, phosphorus inputs are partitioned into nonpoint and point source loads. The point source phosphorus load decreased about 75% during the 1990's study period primarily due to upgrades at the largest municipal wastewater treatment plant in the watershed. Annual nonpoint source phosphorus loads were variable over the study period: dissolved phosphorus ranged from 6,600 kg · y−1 to 20,800 kg · y−1, while particulate phosphorus ranged from 7,400 kg · y−1 to 115,000 kg · y−1. Most of the annual loads of nonpoint source phosphorus, sediment and ammonia were delivered during runoff events. The loads determined through event-based sampling are compared to loads calculated using two other load estimation approaches: an export coefficient technique and fixed frequency sampling combined with the loading software FLUX. The natural variation in nonpoint source loads is examined in the context of setting management goals for improving water quality.

The effects of river input on the ecosystem dynamics in the continental coastal zone of the North Sea using ERSEM

The general characteristics of the continental coastal zone, with nutrient concentrations, primary production and biomass high near the coast but decreasing with distance from the coast, are simulated by a box-refined version of the ecosystem model ERSEM. Aggregated model results compared to the literature as well as to two different three-dimensional models show a good agreement in the coastal region. The dynamical interactions as simulated by the ecosystem model are presented in the form of N P ratios, the limitation by various nutrients and changes in the pathways of the flow of matter in the boxes; e.g. while the silicate limitation stops the spring bloom offshore, near the coast it is terminated by zooplankton grazing. When the river load was reduced by 50%, the largest effect was observed in the coastal boxes with 15% reduction of the net primary production. The discharges of the major rivers hardly affect the central North Sea, but lead to significant changes in nutrient limitations and mass flows in the coastal area. The realistic forcing, which was adopted for this setup, allows a higher net primary production in the southern North Sea in 1989 than in 1988, even though the nutrient river loads in 1989 were lower. The reason appears to be a higher solar energy input in 1989, by about 10 W m −2 d- −1, compared to 1988.

The European regional seas ecosystem model, a complex marine ecosystem model

This paper presents an overview of the concept, structure and implementation of the European Regional Seas Ecosystem Model (ERSEM). The model dynamically simulates the biogeochemical seasonal cycling of carbon, nitrogen, phosphorus and silicon in the pelagic and benthic food webs of the North Sea, and is forced by irradiance, temperature and transport processes. The model has a coarse spatial resolution into ten boxes, the ICES boxes, of which the five deepest have been resolved into surface (0 to 30 m) and deep (30 m to bottom) boxes. At the open boundaries, time series are prescribed for dissolved and particulate nutrients. River loads of nutrients for the rivers discharging into the North Sea are prescribed at monthly intervals. A general circulation model has been used to aggregate the exchange volumes across the box boundaries into daily in- and outflows. From these, the horizontal transports of dissolved and suspended constituents are calculated. Vertical transport is in the form of sinking and sedimentation for particulates and in the form of turbulent diffusion for dissolved constituents. The physical model contains all information specific to the area to be modelled, whereas the biological/chemical submodels have been constructed not to be site-specific. The biological variables are represented as functional groups expressed in units of organic carbon and the chemical variables as the internal pools in the biological variables and as the dissolved inorganic pools in water and sediment, expressed in units of N, P and Si. The model runs in a software environment (SESAME) developed for enabling the development of large and complex models in a modular way by a consortium of institutes, each focusing on different. aspects of the ecosystem, translating these into modules within the model. With the exception of fish populations, where size- and age-structure are explicity represented, all the other biological components have been modelled as unstructured populations aggregated into functional groups. This approach is shown to be appropriate for taxa having short generation times in relation to the annual cycle and for taxa which do not span more than one trophic level during their lifetime.

Ecological research on the animal communities of the Po River Delta lagoons

Abstract The main results of research on invertebrate biology and ecology in the lagoons of the Po River Delta (Sacca del Canarin, Sacca di Scardovari, Sacca di Goro) are presented and related to the more significant approaches followed in ecological research during the last two decades. The lagoons, which are intrinsically eutrophic water bodies, have recently undergone intense eutrophication due to the dramatic increase in the Po river nutrient loading. A shift towards a dystrophic state was indicated by the appearance of large Ulva rigida beds in spring and by extensive anoxia during the summer due to the decomposition of the huge macroalgal biomass. Particularly severe dystrophic crises occurred in the Sacca di Goro. Zooplankton and zoobenthos changes were analyzed according to the variations in hydrographic features as well as those in the trophic state of the water. The significance of population dynamics studies in evaluating biological responses to environmental stresses is emphasized. Investigati...

The Mechanism Controlling Plant Nutrient Concentrations in the Northern Adriatic Sea

AbstractA 20 year data set for the northern Adriatic was analyzed and the factors establishing the nutrient environment identified. Concentrations ranged widely (TIN 0.0–78, PO2 0.01–1.1, and SiO4 0.0–59 mmol m−3). In early winter remineralization increased concentrations. Characteristic winter, late spring and fall phytoplankton blooms alternately decreased and increased concentrations, as modified by river input. In summer nutrients were minimal under a semi‐closed circulation pattern and high vertical stability, due to closely coupled nitrogen and phosphorus assimilation‐regeneration processes and biogenic silica sedimentation. “New” primary production supported mainly by river input of “new” nutrients approximated “regenerated” primary production supported by regenerated nutrients, making the ecosystem especially sensitive to eutrophication pressure from anthropogenic increases in the Po River nutrient load.