Scott W. Nixon

COASTAL MARINE EUTROPHICATION: A DEFINITION, SOCIAL CAUSES, AND FUTURE CONCERNS

Abstract There is a need in the marine research and management communities for a clear operational definition of the term, eutrophication. I propose the following: This definition is consistent with historical usage and emphasizes that eutrophication is a process, not a trophic state. A simple trophic classification for marine systems is also proposed: Various factors may increase the supply of organic matter to coastal systems, but the most common is clearly nutrient enrichment. The major causes of nutrient enrichment in coastal areas are associated directly or indirecdy with meeting the requirements and desires of human nutrition and diet. The deposition of reactive nitrogen emitted to the atmosphere as a consequence of fossil fuel combustion is also an important anthropogenic factor. The intensity of nitrogen emission from fertilizer, livestock waste, and fossil fuel combustion varies widely among the countries of the world. It is strongest in Europe, the northeastern United States, India/Pakistan, Jap...

The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean

Five large rivers that discharge on the western North Atlantic continental shelf carry about 45% of the nitrogen (N) and 70% of the phosphorus (P) that others estimate to be the total flux of these elements from the entire North Atlantic watershed, including North, Central and South America, Europe, and Northwest Africa. We estimate that 61 · 109 moles y−1 of N and 20 · 109 moles y−1 of P from the large rivers are buried with sediments in their deltas, and that an equal amount of N and P from the large rivers is lost to the shelf through burial of river sediments that are deposited directly on the continental slope. The effective transport of active N and P from land to the shelf through the very large rivers is thus reduced to 292 · 109 moles y−1 of N and 13 · 109 moles y−1 of P.The remaining riverine fluxes from land must pass through estuaries. An analysis of annual total N and total P budgets for various estuaries around the North Atlantic revealed that the net fractional transport of these nutrients through estuaries to the continental shelf is inversely correlated with the log mean residence time of water in the system. This is consistent with numerous observations of nutrient retention and loss in temperate lakes. Denitrification is the major process responsible for removing N in most estuaries, and the fraction of total N input that is denitrified appears to be directly proportional to the log mean water residence time. In general, we estimate that estuarine processes retain and remove 30–65% of the total N and 10–55% of the total P that would otherwise pass into the coastal ocean. The resulting transport through estuaries to the shelf amounts to 172–335 · 109 moles y−1 of N and 11–19 · 109 moles y−1 of P. These values are similar to the effective contribution from the large rivers that discharge directly on the shelf.For the North Atlantic shelf as a whole, N fluxes from major rivers and estuaries exceed atmospheric deposition by a factor of 3.5–4.7, but this varies widely among regions of the shelf. For example, on the U.S. Atlantic shelf and on the northwest European shelf, atmospheric deposition of N may exceed estuarine exports. Denitrification in shelf sediments exceeds the combined N input from land and atmosphere by a factor of 1.4–2.2. This deficit must be met by a flux of N from the deeper ocean. Burial of organic matter fixed on the shelf removes only a small fraction of the total N and P input (2–12% of N from land and atmosphere; 1–17% of P), but it may be a significant loss for P in the North Sea and some other regions. The removal of N and P in fisheries landings is very small. The gross exchange of N and P between the shelf and the open ocean is much larger than inputs from land and, for the North Atlantic shelf as a whole, it may be much larger than the N and P removed through denitrification, burial, and fisheries. Overall, the North Atlantic continental shelf appears to remove some 700–950· 109 moles of N each year from the deep ocean and to transport somewhere between 18 and 30 · 109 moles of P to the open sea. If the N and P associated with riverine sediments deposited on the continental slope are included in the total balance, the net flux of N to the shelf is reduced by 60 · 109 moles y−1 and the P flux to the ocean is increased by 20 · 109 moles y−1. These conclusions are quite tentative, however, because of large uncertainties in our estimates of some important terms in the shelf mass balance.

Between Coastal Marshes and Coastal Waters — A Review of Twenty Years of Speculation and Research on the Role of Salt Marshes in Estuarine Productivity and Water Chemistry

It has been almost 20 years since John Teal (1962) published his well-known paper synthesizing a variety of independent studies of production, respiration, and animal abundances in the salt marsh ecosystem of Sapelo Island, Georgia. Teal’s work brought out a number of interesting points, but I think the reason the paper is most often cited is because of its last sentence. After discussing various trophic relationships in the marsh, the paper ended with the conclusion that “...the tides remove 45% of the production before the marsh consumers have a chance to use it and in so doing permit the estuaries to support an abundance of animals.”

Remineralization and Nutrient Cycling in Coastal Marine Ecosystems

Our views of remineralization and nutrient cycling in coastal marine ecosystems have changed considerably over the last 30 years. The major trend has been an increasing appreciation for the complexity of processes involved, including some marked changes in our assessment of the importance of bacteria with respect to smaller animals and in our perception of the association between bacteria and particulate matter in the sea. Among the more recent developments in this area is a growing awareness of the importance of the coupling between benthic and pelagic communities in coastal waters. There appears to be a strong linear correlation between the organic matter produced in the overlying water and the amount of organic matter consumed on the bottom in almost all of the coastal environments for which annual data are available. The large amount of organic matter consumed by the benthos (perhaps 25–50 percent of that produced) is associated with a large flux of inorganic nutrients from the sediments to the overlying water. The stoichiometry of net benthic nutrient regeneration differs from that of pelagic regeneration, however, and simple Redfield type models probably cannot be applied. The amount of fixed inorganic nitrogen returned to the water across the sediment-water interface appears to be about half of that expected on the basis of the flux of phosphorus. This behavior, along with the fact that an appreciable amount of organic matter in coastal waters gets remineralized on the bottom, contributes to the low N/P ratio that is characteristic of these areas and may be responsible for the observation that nitrogen is commonly the nutrient most limiting for primary production. Recent direct measurements of the flux of dissolved N2 across the sediment-water interface indicate that denitrification is probably responsible for the loss of fixed nitrogen during decomposition in the sediments. If this is a widespread phenomenon, estuaries, bays, and other coastal waters may be major sinks in the marine nitrogen cycle and important terms in the global nitrogen budget. However, the fact that eutrophication appears to be an increasing problem in many estuaries is dramatic warning that anthropogenic nutrient inputs can overwhelm the recycling and remineralization processes in coastal waters.

NUTRIENTS AND THE PRODUCTIVITY OF ESTUARINE AND COASTAL MARINE ECOSYSTEMS

SUMMARY Recent research on estuarine and coastal marine systems has revealed two particularly interesting things about nutrients and productivity. First is the observation that these areas are among the most intensively fertilized environments on earth. Second is the common finding that much of the characteristically high primary productivity of these shallow waters is supported by nutrients released or recycled by pelagic and benthic microheterotrophs. Since nutrient inputs to coastal areas have probably been increasing and are likely to continue to do so, it is particularly important to understand the relationship between nutrient loading and nutrient cycling and the extent to which their interactions may set the levels of primary and secondary production in coastal systems. That some direct relationship exists between the input of nutrients and the productivity of higher trophic levels has been a principle of marine ecology since the turn of the century. It is surprisingly difficult, however, to find q...

A Strikingly Rich Zone-Nutrient Enrichment and Secondary Production in Coastal Marine Ecosystems

Despite a recent review concluding that there is little or no reason to expect that the production of fish and other animals will increase with nutrient enrichment or eutrophication, there is a variety of evidence that anthropogenic nutrients can stimulate secondary production in marine ecosystems. Unique multiple-year fertilization experiments were carried out over fifty years ago in Scottish sea lochs that showed dramatic increases in the abundance of benthic infauna and greatly enhanced growth of fish as a result of inorganic nitrogen (N) and phosphorus (P) additions. These experiments appear to have provided a good qualitative model for the responses of the Baltic Sea to nutrient enrichment and resulting eutrophication. Historical comparisons by others have shown that the weight of benthic animals per unit area above the halocline in the Baltic is now up to 10 or 20 times greater than it was in the early 1920s and that the total fish biomass in the system may have increased 8 fold between the early part of the 1900s and the 1970s. While there are no similar data for the highly enriched central and southern North Sea, there is convincing evidence that the growth rates of plaice, sole, and other species have increased there since the 1960s or 1970s. Cross-system comparisons have also shown that there are strong correlations between primary production and the production and yield of fish and the standing crop and production of benthic macrofauma in phytoplankton-dominated marine ecosystems. Concerns over the growing nutrient (especially N) enrichment of coastal marine waters are clearly valid and deserve the attention of scientists and managers, but the recent demonizing of N ignores the fact that nutrients are a fundamental requirement for producing biomass. Decisions regarding the amount of N or P that will be allowed to enter marine ecosystems should be made with the full knowledge that there may be tradeoffs between increases in water clarity and dissolved oxygen and the abundance of oysters, clams, fish, and other animals we desire.

Accretion rates and sediment accumulation in Rhode Island salt marshes

In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,210Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr−1 (0.25 to 0.60 cm yr−1) based on the constant flux model, 0.40±0.15 cm yr−1 (0.15 to 0.58 cm yr−1) based on the constant activity model, and 0.44±0.11 cm yr−1 (0.30 to 0.59 cm yr−1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess210Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr−1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.

An assessment of the annual mass balance of carbon, nitrogen, and phosphorus in Narragansett Bay*

Narragansett Bay is a relatively well-mixed, high salinity coastal embayment and estuary complex in southern New England (USA). Much of the shoreline is urban and the watershed is densely developed. We have combined our data on C, N, and P inputs to this system, on C, N, and P accumulation in the sediments, and on denitrification with extensive work by others to develop approximate annual mass balances for these elements. The results show that primary production within the bay is the major source of organic carbon (4 times greater than other sources), that land drainage and upstream sewage and fertilizer are the major sources of N, and that landward flowing bottom water from offshore may be a major source of dissolved inorganic phosphorus. Most of the nutrients entering the bay arrive in dissolved inorganic form, though DON is a significant component of the N carried by the rivers. About 40% of the DIN in the rivers is in the form of ammonia. Sedimentation rates are low in most of Narragansett Bay, and it appears that less than 20% of the total annual input of each of these elements is retained within the system. A very small amount of C, N, and P is removed in fisheries landings, denitrification in the sediments removes perhaps 10–25% of the N input, and most of the carbon fixed in the system is respired within it. Stoichiometric calculations suggest that some 10–20% of the organic matter formed in the bay is exported to offshore and that Narragansett Bay is an autotrophic system. Most of the N and P that enters the bay is, however, exported to offshore waters in dissolved inorganic form. This assessment of the overall biogeochemical behavior of C, N, and P in the bay is consistent with more rigorously constrained mass balances obtained using large living models or mesocosms of the bay at the Marine Ecosystem Research Laboratory (MERL).

EFFECTS OF THE SPAWNING MIGRATION OF THE ALEWIFE, ALOSA PSEUDOHARENGUS, ON FRESHWATER ECOSYSTEMS'

The influx of large numbers of alewife, Alosa pseudoharen gus, into relatively small freshwater systems may have a considerable impact upon pre-established food chains and nutrient cycles. We estimate the total nutrient input to Pausacaco Pond, Rhode Island, USA, from alewives amounted to 0.43 g P. 2.7 g N, and 16.8 g C/M2 over a 2-mo period. This is largely through mortality of the spawning fish, and to a lesser extent through excretion. These inputs were much greater than the eventual nutrient loss to the system through emigration of juvenile fish. In tank experiments using pond microcosms, the initial response to the addition of the fish was a large phytoplankton bloom and an increase in litter respiration. The phytoplankton bloom was short- lived, and the most lasting effect was an increase in production and respiration in the leaf litter. This increased production in the litter community would support a long lasting supply of insect and benthic invertebrate food for young fish. The respiration rate of autumn leaves incubated in alewife streams during the migration was significantly higher than that of leaves incubated simultaneously in a stream which had no alewife run. Respiration rates of leaves incubated in the same streams before the arrival of alewives did not differ significantly. The increase in litter respiration, an indication of microbial and invertebrate activity on the leaf surface, was attributed to the additional nutrients supplied by the fish.

Reversal of the net dinitrogen gas flux in coastal marine sediments

The flux of nitrogen from land and atmosphere to estuaries and the coastal ocean has increased substantially in recent decades. The observed increase in nitrogen loading is caused by population growth, urbanization, expanding water and sewer infrastructure, fossil fuel combustion and synthetic fertilizer consumption. Most of the nitrogen is removed by denitrification in the sediments of estuaries and the continental shelf, leading to a reduction in both cultural eutrophication and nitrogen pollution of the open ocean. Nitrogen fixation, however, is thought to be a negligible process in sub-tidal heterotrophic marine systems. Here we report sediment core data from Narragansett Bay, USA, which demonstrate that heterotrophic marine sediments can switch from being a net sink to being a net source of nitrogen. Mesocosm and core incubation experiments, together with a historic data set of mean annual chlorophyll production, support the idea that a climate-induced decrease in primary production has led to a decrease in organic matter deposition to the benthos and the observed reversal of the net sediment nitrogen flux. Our results suggest that some estuaries may no longer remove nitrogen from the water column. Instead, nitrogen could be exported to the continental shelf and the open ocean and could shift the effect of anthropogenic nitrogen loading beyond the immediate coastal zone.