A primary focus of coastal science during the past 3 decades has been the question: How does anthropogenic nutrient enrichment cause change in the structure or function of nearshore coastal ecosystems? This theme of environmental science is recent, so our conceptual model of the coastal eutrophication problem continues to change rapidly In this review, I suggest that the early (Phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophication by the 1960s The Phase I model emphasized changing nutrient input as a signal, and responses to that signal as increased phytoplankton biomass and primary production, decomposition of phytoplankton- derived organic matter, and enhanced depletion of oxygen from bottom waters Coastal research in recent decades has identified key differences in the responses of lakes and coastal-estuarine ecosystems to nutrient enrichment The contemporary (Phase II) conceptual model reflects those differences and includes explicit recognition of (1) system-specific attributes that act as a filter to modulate the responses to enrichment (leading to large differences among estuarine-coastal systems in their sensitivity to nu- trient enrichment); and (2) a complex suite of direct and indirect responses including linked changes in: water transparency, distribution of vascular plants and biomass of macroalgae, sediment biogeochem- istry and nutrient cycling, nutrient ratios and their regulation of phytoplankton community composition, frequency of toxic/harmful algal blooms, habitat quality for metazoans, reproduction/growth/survival of pelagic and benthic invertebrates, and subtle changes such as shifts in the seasonality of ecosystem functions Each aspect of the Phase II model is illustrated here with examples from coastal ecosystems around the world In the last section of this review I present one vision of the next (Phase III) stage in the evolution of our conceptual model, organized around 5 questions that will guide coastal science in the early 21st century: (1) How do system-specific attributes constrain or amplify the responses of coastal ecosystems to nutrient enrichment? (2) How does nutrient enrichment interact with other stressors (toxic contaminants, fishing harvest, aquaculture, nonindigenous species, habitat loss, climate change, hydro- logic manipulations) to change coastal ecosystems? (3) How are responses to multiple stressors linked? (4) How does human-induced change in the coastal zone impact the Earth system as habitat for humanity and other species? (5) How can a deeper scientific understanding of the coastal eutrophication problem be applied to develop tools for building strategies at ecosystem restoration or rehabilitation?

https://www.int-res.com/articles/meps/210/m210p223.pdf

Our evolving conceptual model of the coastal eutrophication problem

Although algal blooms, including those considered toxic or harmful, can be natural phenomena, the nature of the global problem of harmful algal blooms (HABs) has expanded both in extent and its public perception over the last several decades. Of concern, especially for resource managers, is the potential relationship between HABs and the accelerated eutrophication of coastal waters from human activities. We address current insights into the relationships between HABs and eutrophication, focusing on sources of nutrients, known effects of nutrient loading and reduction, new understanding of pathways of nutrient acquisition among HAB species, and relationships between nutrients and toxic algae. Through specific, regional, and global examples of these various relationships, we offer both an assessment of the state of understanding, and the uncertainties that require future research efforts. The sources of nutrients poten- tially stimulating algal blooms include sewage, atmospheric deposition, groundwater flow, as well as agricultural and aquaculture runoff and discharge. On a global basis, strong correlations have been demonstrated between total phos- phorus inputs and phytoplankton production in freshwaters, and between total nitrogen input and phytoplankton pro- duction in estuarine and marine waters. There are also numerous examples in geographic regions ranging from the largest and second largest U.S. mainland estuaries (Chesapeake Bay and the Albemarle-Pamlico Estuarine System), to the Inland Sea of Japan, the Black Sea, and Chinese coastal waters, where increases in nutrient loading have been linked with the development of large biomass blooms, leading to anoxia and even toxic or harmful impacts on fisheries re- sources, ecosystems, and human health or recreation. Many of these regions have witnessed reductions in phytoplankton biomass (as chlorophyll a) or HAB incidence when nutrient controls were put in place. Shifts in species composition have often been attributed to changes in nutrient supply ratios, primarily N:P or N:Si. Recently this concept has been extended to include organic forms of nutrients, and an elevation in the ratio of dissolved organic carbon to dissolved organic nitrogen (DOC:DON) has been observed during several recent blooms. The physiological strategies by which different groups of species acquire their nutrients have become better understood, and alternate modes of nutrition such as heterotrophy and mixotrophy are now recognized as common among HAB species. Despite our increased un- derstanding of the pathways by which nutrients are delivered to ecosystems and the pathways by which they are assimilated differentially by different groups of species, the relationships between nutrient delivery and the development of blooms and their potential toxicity or harmfulness remain poorly understood. Many factors such as algal species presence/ abundance, degree of flushing or water exchange, weather conditions, and presence and abundance of grazers contribute to the success of a given species at a given point in time. Similar nutrient loads do not have the same impact in different environments or in the same environment at different points in time. Eutrophication is one of several mechanisms by which harmful algae appear to be increasing in extent and duration in many locations. Although important, it is not the only explanation for blooms or toxic outbreaks. Nutrient enrichment has been strongly linked to stimulation of some harmful species, but for others it has not been an apparent contributing factor. The overall effect of nutrient over- enrichment on harmful algal species is clearly species specific.

/pdf/harmful-algal-blooms-and-eutrophication-nutrient-sources-4ujytujxjq.pdf

Harmful Algal Blooms and Eutrophication: Nutrient Sources, Composition, and Consequences

This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophi- cation in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macro- infauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively un- affected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay's overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest funda- mental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others.

/pdf/eutrophication-of-chesapeake-bay-historical-trends-and-1k0ro6bolk.pdf

Eutrophication of Chesapeake Bay: historical trends and ecological interactions

The second largest zone of coastal hypoxia (oxygen-depleted waters) in the world is found on the northern Gulf of Mexico continental shelf adjacent to the outflows of the Mississippi and Atchafalaya Rivers. The combination of high freshwater discharge, wind mixing, regional circulation, and summer warming controls the strength of stratification that goes through a well-defined seasonal cycle. The physical structure of the water column and high nutrient loads that enhance primary production lead to an annual formation of the hypoxic water mass that is dominant from spring through late summer. Paleoindicators in dated sediment cores indicate that hypoxic conditions likely began to appear around the turn of the last century and became more severe since the 1950s as the nitrate flux from the Mississippi River to the Gulf of Mexico tripled. Whereas increased nutrients enhance the production of some organisms, others are eliminated from water masses (they either emigrate from the area or die) where the...

Gulf of Mexico Hypoxia, A.K.A. “The Dead Zone”

. Water masses can become undersaturated with oxygen when natural processes alone or in combination with anthropogenic processes produce enough organic carbon that is aerobically decomposed faster than the rate of oxygen re-aeration. The dominant natural processes usually involved are photosynthetic carbon production and microbial respiration. The re-supply rate is indirectly related to its isolation from the surface layer. Hypoxic water masses ( Hypoxia development and continuation in many areas of the world's coastal ocean is accelerated by human activities, especially where nutrient loading increased in the Anthropocene. This higher loading set in motion a cascading set of events related to eutrophication. The formation of hypoxic areas has been exacerbated by any combination of interactions that increase primary production and accumulation of organic carbon leading to increased respiratory demand for oxygen below a seasonal or permanent pycnocline. Nutrient loading is likely to increase further as population growth and resource intensification rises, especially with increased dependency on crops using fertilizers, burning of fossil fuels, urbanization, and waste water generation. It is likely that the occurrence and persistence of hypoxia will be even more widespread and have more impacts than presently observed. Global climate change will further complicate the causative factors in both natural and human-caused hypoxia. The likelihood of strengthened stratification alone, from increased surface water temperature as the global climate warms, is sufficient to worsen hypoxia where it currently exists and facilitate its formation in additional waters. Increased precipitation that increases freshwater discharge and flux of nutrients will result in increased primary production in the receiving waters up to a point. The interplay of increased nutrients and stratification where they occur will aggravate and accelerate hypoxia. Changes in wind fields may expand oxygen minimum zones onto more continental shelf areas. On the other hand, not all regions will experience increased precipitation, some oceanic water temperatures may decrease as currents shift, and frequency and severity of tropical storms may increase and temporarily disrupt hypoxia more often. The consequences of global warming and climate change are effectively uncontrollable at least in the near term. On the other hand, the consequences of eutrophication-induced hypoxia can be reversed if long-term, broad-scale, and persistent efforts to reduce substantial nutrient loads are developed and implemented. In the face of globally expanding hypoxia, there is a need for water and resource managers to act now to reduce nutrient loads to maintain, at least, the current status.

/pdf/dynamics-and-distribution-of-natural-and-human-caused-21e6uqwqxc.pdf

Dynamics and distribution of natural and human-caused hypoxia

In this paper we assemble and analyze quantitative annual input-export budgets for total nitrogen (TN) and total phosphorus (TP) for Chesapeake Bay and three of its tributary estuaries (Potomac, Patuxent, and Choptank rivers). The budgets include estimates of TN and TP sources (point, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cycling rates (zooplankton excretion and net sediment-water flux), and net downstream exchange. Annual terrestrial and atmospheric inputs (average of 1985 and 1986 data) of TN and TP ranged from 4.3 g TN m−2 yr−1 to 29.3 g TN m−2 yr−1 and 0.32 g TP m−2 yr−1 to 2.42 g TP m−2 yr−1, respectively. These rates of TN and TP input represent 6-fold to 8-fold and 13-fold to 24-fold increases in loads to these systems since the precolonial period. A recent 11-yr record for the Susquehanna River indicates that annual loads of TN and TP have varied by about 2-fold and 4-fold, respectively. TN inputs increased and TP inputs decreased during the 11-yr period. The relative importance of nutrient sources varied among these estuaries: point sources of nutrients delivered about half the annual TN and TP load to the Patuxent and nearly 60% of TP inputs to the Choptank; diffuse sources contributed 60–70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet-fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct, although damped, relationships between annual rates of nutrient input, water-column and sediment nutrient stocks, and nutrient losses via burial in sediments and denitrification. Our budgets indicate that the annual mass balance of TN and TP is maintained by a net landward exchange of TP and, with one exception (Choptank River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terrestrial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the literature were resolved by the inclusion of sediments derived from shoreline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmospheric dry-fall, contributions of nutrients via groundwater, and the sedimentation rates used to calculate nutrient burial rates.

Inputs, transformations, and transport of nitrogen and phosphorus in Chesapeake Bay and selected tributaries

External nutrient loadings, internal nutrient pools, and phytoplankton production were examined for three major subsystems of the Chesapeake Bay Estuary--the upper Mainstem, the Patuxent Estuary, and the Potomac Estuary---during 1985-1989. The atomic nitrogen to phosphorus ratios (TN:TP) of total loads to the Mainstem, Patuxent, and the Potomac were 51, 29 and 35, respectively. Most of these loads entered at the head of the estuaries from riverine sources and major wastewater treatment plants. Approximately 7-16% of the nitrogen load entered the head of each estuary as particulate matter in contrast to 48-69% for phosphorus. This difference is hypothesized to favor a greater loss of phosphorus than nitrogen through sedimentation and burial. This process could be important in driving estuarine nitrogen to phosphorus ratios above those of inputs. Water column TN: TP ratios in the tidal fresh, oligohaline, and mesohaline salinity zones of each estuary ranged from 56 to 82 in the Mainstem, 27 to 48 in the Patuxent, and 72 to 126 in the Potomac. A major storm event in the Potomac watershed was shown to greatly increase the particulate fraction of nitrogen and phosphorus and lower the TN:TP in the river-borne loads. The load during the month that contained this storm (November 1985) accounted for 11% of the nitrogen and 31% of the phosphorus that was delivered to the estuary by the Potomac River during the entire 60- month period examined here. Within the Mainstem estuary, salinity dilution plots revealed strong net sources of ammonium and phosphate in the oligohaline to upper mesohaline region, indicating that these areas were sites of considerable internal recycling of nutrients to surface waters. The sedimentation of particulate nutrient loads in the upper reaches of the estuary is probably a major source of these recycled nutrients. A net sink of nitrate was indicated during summer. A combination of inputs and these internal recycling processes caused dissolved inorganic N to P ratios to approach 16:1 in the mesohaline zone of the Mainstem during late summer; this ratio was much higher at other times and in the lower salinity zones. Phytoplankton biomass in the mesohaline Mainstem reached a peak in spring and was relatively constant throughout the other seasons. Productivity was highest in spring and summer, accounting for approximately 33% and 44%, respectively, of the total annual productivity in this region. In the Patuxent and Potomac, the TN:TP ratios of external loads documented here are 2--4 times higher than those observed over the previous two decades. These changes are attributed to point-source phosphorus controls and the likelihood that nitrogen-rich nonpoint source inputs, including contributions from the atmosphere, have increased. These higher N:P ratios relative to Redfield proportions (16:1) now suggest a greater overall potential for phos- phorus-limitatio n rather than nitrogen-limitat ion of phytoplankton in the areas studied.

External Nutrient Sources, Internal Nutrient Pools, and Phytoplankton Production in

External nutrient loadings, internal nutrient pools, and phytoplankton production were examined for three major subsystems of the Chesapeake Bay Estuary—the upper Mainstem, the Patuxent Estuary, and the 01 Potomac Estuary—during 1985–1989. The atomic nitrogen to phosphorus ratios (TN:TP) of total loads to the 01 Mainstem, Patuxent, and the Potomac were 51, 29 and 35, respectively. Most of these loads entered at the head of the estuaries from riverine sources and major wastewater treatment plants. Approximately 7–16% for the nitrogen load entered the head of each estuary as particulate matter in contrast to 48–69% for phosphorus. This difference is hypothesized to favor a greater loss of phosphorus than nitrogen through sedimentation and burial. This process could be important in driving estuarine nitrogen to phosphorus ratios above those of inputs. Water column TN: TP ratios in the tidal fresh, oligohaline, and mesohaline salinity zones of each estuary ranged from 56 to 82 in the Mainstem, 27 to 48 in the Patuxent, and 72 to 126 in the Potomac. A major storm event in the Potomac watershed was shown to greatly increase the particulate fraction of nitrogen and phosphorus and lower the TN:TP in the river-borne loads. The load during the month that contained this storm (November 1985) accounted for 11% of the nitrogen and 31% of the phosphorus that was delivered to the estuary by the Potomac River during the entire 60-month period examined here. Within the Mainstem estuary, salinity dilution plots revealed strong net sources of ammonium and phosphate in the oligohaline to upper mesohaline region, indicating that these areas were sites of considerable internal recycling of nutrients to surface waters. The sedimentation of particulate nutrient loads in the upper reaches of the estuary is probably a major source of these recycled nutrients. A net sink of nitrate was indicated during summer. A combination of inputs and these internal recycling processes caused dissolved inorganic N to P ratios to approach 16:1 in the mesohaline zone of the Mainstem during late summer; this ratio was much higher at other times and in the lower salinity zones. Phytoplankton biomass in the mesohaline Mainstem reached a peak in spring and was relatively constant throughout the other seasons. Productivity was highest in spring and summer, accounting for approximately 33% and 44%, respectively, of the total annual productivity in this region. In the Patuxent and Potomac, the TN:TP ratios of external loads documented here are 2–4 times higher than those observed over the previous two decades. These changes are attributed to point-source phosphorus controls and the likelihood that nitrogen-rich nonpoint source inputs, including contributions from the atmosphere, have increased. These higher N:P ratios relative to Redfield proportions (16:1) now suggest a greater overall potential for phosphorus-limitation rather than nitrogen-limitation of phytoplankton in the areas studied.

External nutrient sources, internal nutrient pools, and phytoplankton production in Chesapeake Bay

ABSTTLACT: In this paper we assemble and analyze quantitative annual inputexport budgets for total nitrogen (TN) and total phosphorus (TP) for Chesapeake Bay and three of its tributary estuaries (Potomac, Patuxent, and Choptank rivers). The budgets include estimates of TN and TP sources (point, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cyclhrg rates (zooplankton excretion and net sediment-water flux), and net downstream exchange. Annual terrestrial and atmospheric inputs (average of 1985 and 1986 data) of TN and TP ranged from 4.3 g TN tn2 yr-’ to 29.3 g.TN rnw2 v-l and 0.32 g TP mm2 yr’ to 2.42 g TP m-* yr’, respectively. These rates of TN and TP input represent 6-fold to f diffuse sources contributed SO-70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet-fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct, although damped, relationships between annual rates of nutrient input, water-column and sediment nutrient stocks, and nutrient losses via burial in sediments and denitrification. Our budgets indicate that the ammal mass balance of TN and TP is maintained by a net Jandward exchange of TP and, with one exception (Choptank River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terrestrial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the literature were resolved by the inclusion of sediments derived from shoreline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmospheric dry-fall, contributions of nutrients via groundwater, and the sedimentation rates used to calculate nutrient burial rates.

Robert M. Summers

Papers

Inputs, transformations, and transport of nitrogen and phosphorus in Chesapeake Bay and selected tributaries

External Nutrient Sources, Internal Nutrient Pools, and Phytoplankton Production in

External nutrient sources, internal nutrient pools, and phytoplankton production in Chesapeake Bay

Inputs, Transformations, and Transport of Nitrogen and Phosphorus in Chesapeake Selected Tributaries Bay and