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Nutrients, eutrophication and harmful algal blooms along the freshwater to marine continuum

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Abstract Agricultural, urban and industrial activities have dramatically increased aquatic nitrogen and phosphorus pollution (eutrophication), threatening water quality and biotic integrity from headwater streams to coastal areas world‐wide. Eutrophication creates multiple problems, including hypoxic “dead zones” that reduce fish and shellfish production; harmful algal blooms that create taste and odor problems and threaten the safety of drinking water and aquatic food supplies; stimulation of greenhouse gas releases; and degradation of cultural and social values of these waters. Conservative estimates of annual costs of eutrophication have indicated $1 billion losses for European coastal waters and $2.4 billion for lakes and streams in the United States. Scientists have debated whether phosphorus, nitrogen, or both need to be reduced to control eutrophication along the freshwater to marine continuum, but many management agencies worldwide are increasingly opting for dual control. The unidirectional flow of water and nutrients through streams, rivers, lakes, estuaries and ultimately coastal oceans adds additional complexity, as each of these ecosystems may be limited by different factors. Consequently, the reduction of just one nutrient upstream to control eutrophication can allow the export of other nutrients downstream where they may stimulate algal production. The technology exists for controlling eutrophication, but many challenges remain for understanding and managing this global environmental problem. This article is categorized under: Science of Water > Water Quality Water and Life > Stresses and Pressures on Ecosystems
Whole‐lake experiments. (a) Response of phytoplankton to nitrogen (+N), phosphorus (+P), or N + P additions in 20 whole‐lake experiments relative to controls (either pretreatment sampling or nearby control lakes). Each lake is indicated by a different number. Some lakes were treated for multiple years (shown sequentially). Although there are many lake‐years of analysis, relatively few lakes have been studied to test N‐alone (4) or P‐alone (9) nutrient responses. Measured, or estimated chlorophyll levels on control lakes ranged from 0.3 to 4.8 μg L−1, with a mean of 1.8 μg L−1, indicating that all had naturally low productivity. Studies were done in the Experimental Lakes area in Ontario, Canada (ELA), northern Sweden, Northwest Territories of Canada (NWT), Norway, Quebec, Alaska, and Kodiak Island (AK). The values over each set of bars are the mean annual responses for each nutrient treatment. Details and references for these studies are given in Appendix S1, Supporting Information. (b) Relationship between phosphorus fertilization rates in the whole‐lake experiments with (n = 8) and without (n = 5) concomitant nitrogen additions. Only a subset of lakes shown in the top frame (a) had sufficient data to calculate addition rates. Some lakes were fertilized for multiple years. Phytoplankton responses were measured as either chlorophyll levels, algal biovolume, or primary production. Both phosphorus level and the presence/absence of nitrogen were significant (p < .000) factors influencing responses. Regression equations: +P, Log Response = −0.301 + 0.257 Log [P]; N + P, Log Response = −13.46 + 10.52 Log [P]. Detailed data are given in Appendix S1
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Relationship between phytoplankton chlorophyll levels (z‐axis) and total phosphorus and total nitrogen levels in 1264 lakes studied in the Environmental Protection Agency's National Lakes Survey (EPA, ). Only lakes from the EPA database where TP was ≥3 μg L−1 were used in the analysis. The surface curve was fit with local regression smoothing (LOESS; SigmaPlot) and a sampling proportion of 0.4. Dodds and Smith () recently reported a very similar relationship between N, P and chlorophyll in benthic periphyton of streams
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Relationship between nitrogen and phosphorus concentrations in 1268 lakes and reservoirs sampled by the EPA (, ) during the National Lakes Assessment (NLA). The red dashed line shows where there is a 14:1 ratio (by weight) between N and P (Downing & McCauley, ). Classification of lake trophic state is from Carlson ()
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Simplified view of how external loading and cycling of nitrogen and phosphorus interact with loss factors for these nutrients to control the size of algal populations and cyanobacteria in inland and coastal waters. Some environmental factors influencing N2 fixation and denitrification are shown. Adapted from (Paerl et al., ). Phosphorus loss from the system is due to advective loss and burial. High sediment concentrations of phosphorous provide “legacy” nutrient loading back into the water column. Although this can also be a problem for nitrogen, denitrification can remove much of the sedimented nitrogen from the system
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(a) Relative increases of total nitrogen and total phosphorus in rivers in 14 Ecoregions of the U.S. values are plotted as response ratios [= ln (current concentration/historic concentration)]. (b) Relative changes in the N:P response ratios in the 14 ecoregions. Derived from Dodds and Smith (). For reference, a response ratio of 1.6 indicates a fivefold increase in N or P or the N:P ratio
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(a) Total nitrogen concentrations in 1894 streams and rivers in the United States (derived from EPA, , ). Total nitrogen can be used as one index of eutrophication (Dodds, ). The pattern of total phosphorus (not shown) in stream waters was similar to that for nitrogen, although more western streams had high phosphorus concentration. (b). Chlorophyll a levels in the in 1264 United States lakes studied in the Environmental Protection Agency's National Lakes Survey (EPA, ). Only lakes from the EPA database where TP was ≥3 μg L−1 were used in the analysis. The trophic state classifications were based on Caspers () for annual mean chlorophyll a levels
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Flow‐weighted mean nutrient concentrations in rivers draining cropland (n = 104), urban (n = 38) and forest (n = 36) watersheds. (a). Nitrogen, showing the most readily bioavailable forms of ammonia and nitrate as well as organic nitrogen. (b). Phosphorus concentrations in readily bioavailable soluble reactive phosphorus (SRP) and other less bioavailable forms. (c). Total N:Total P ratio (by weight). The nutrient data were derived from a U.S. Geological Survey NAWQA survey (USGS, )
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(a) Filamentous algae (Cladophora sp.) in a nutrient‐impacted Greenbrier River, West Virginia. (b). Bloom of the cyanobacterium, Nodularia spumigena in Farmington Bay of the Great Salt Lake (Utah, USA). This toxic species creates dangerous blooms in saline estuaries such as the Baltic Sea and Gippsland Lakes, Australia. Photo credits: a) West Virginia Department of Environmental Protection; b) Wayne Wurtsbaugh
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Relationship between total phosphorus and phytoplankton concentrations in 292 streams and rivers primarily in North America (79%; especially tributaries of the Missouri and lower Mississippi Rivers) and Europe (16%). Data were derived from the data set of Van Nieuwenhuyse and Jones (1996). Log Chl = −1.65 + 1.99 log TP—0.28 (Log TP)2. Dotted lines show suggested boundaries between oligotrophic, mesotrophic and eutrophic river conditions (Dodds et al., )
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Publications referenced in web of science™ for five‐year periods beginning in the indicated year. Search terms used in combination with “eutrophication” were: Lakes—“pond*, lake*, reservoir* or lentic”; estuaries and coastal—“marine, coastal, estuar*, ocean, sea”; streams and Rivers—“creek*, stream*, river*, lotic”. The black dotted line shows the scaled increase since 1960 in publications found using the search term “ecolog*”
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Relationship between total nitrogen in water and fish biomass in 48 Florida lakes (USA). One statistical outlier (triangle) was removed from the regression. Fish biomass was less closely correlated with total phosphorus (r2 = 0.34, p = .009). Derived from data of Bachmann et al. ()
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A hypothetical watershed showing the linkages among streams, lakes and coastal zones. Pollution sources for nitrogen and phosphorus, and approximate water retention times that influence eutrophication are shown. Nitrogen fixed by cyanobacteria can accumulate in systems with long residence times but will be flushed out in systems with short retention times. Movement of these two nutrients through complex watersheds and into marine systems complicates nutrient control strategies to reduce eutrophication
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MODIS satellite image of a harmful algal bloom in Lake Erie on September 29, 2014. Data from NASA's Aqua satellite. https://www.noaa.gov/stories2015/images/aqua.2014272.0929.1845C.L3.LE3.v670.truecolor_logos.png
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