Greater input of growth limiting nutrient
More filamentous
algae
Higher concentration of that nutrient in the water
Increased phytoplankton production and algal blooms Less light penetration Less macroalgae and seagrass Increased sedimentation of organic matter
More fish above the oxycline
More benthic animals above the oxycline
Fewer fish below, and potentially above,
the oxycline Increased zooplankton
Oxygen deficiency & hydrogen sulfide formation
Reduced demersal & benthic abundance and diversity Less suitable habitat for
feeding, predator avoidance and
reproduction
Figure 5.1 Series of responses within a coastal ecosystem to the increased input of a limiting nutrient. Green processes indicate eutrophication. Blue processes or conditions indicate increased secondary production. Red processes or conditions indicate the negative effects of eutrophication. (See insert for color representation.)
5.3 SYMPTOMS
The initial response of an estuarine or coastal system to an increase in limiting nutrients is an increase in phytoplankton growth rate and biomass accumulation, growth of filamentous macroalgae, blooms of noxious or toxic algae, reduction in water clarity, shifts in phytoplankton community structure, or combinations of these (Figure 5.1). 5.3.1 Shifts in Phytoplankton Communities
Phytoplankton are affected not only by the quantity of nutrient loading but also by the relative supply of nutrients. Global patterns of the ratios of dissolved nitrogen, phos- phorus, and silica in large rivers indicate that primarily nitrate flux controls the ratios of these nutrients as delivered to the coastal ocean (Turner et al., 2003a). As the N/P ratio rises above the Redfield ratio of 16:1, phosphorus limitation of phytoplankton growth is implied, and in a similar fashion a Si/N ratio below 1:1 implies silica limi- tation (Turner et al., 2003b). Results from field and laboratory studies have suggested that the lack of silica or phosphorus relative to nitrogen can control phytoplankton community composition (e.g., Rabalais et al., 1996, Dortch et al., 2001).
A series of eutrophication-related ecological changes in the Baltic Sea and Kattegat were caused primarily by increased anthropogenic nutrient inputs (both nitrogen and phosphorus) mainly after World War II (reviewed by Elmgren and Larsson, 2001). One of the system responses was an increase in toxic or noxious algal blooms. Cyanobacterial blooms in the open sea, particularly of the toxic, nitrogen-fixing genus Nodularia, are the main problem. However, a number of fish kills by the prymnesiophyte Prymnesium pravum have been reported from the Baltic proper
SYMPTOMS 121
coastal zone. The former is stimulated by low inorganic N/P ratios; the latter is thought to be favored by high N/P ratios.
As the Si/N ratio declines as a response to increased nitrogen, reduced silicon, or both (Turner et al., 2003a), a phytoplankton community of nondiatoms may be competitively enabled (Officer and Ryther, 1980). This alternative community would be more likely to be composed of flagellated algae, especially dinoflagellates, including noxious bloom-forming algal communities. They argued further that the fisheries web would re-form and be composed of less desirable species. Evidence for shifts in the phytoplankton community in the northern Gulf of Mexico indicates a change from heavily silicified diatoms to less silicified diatoms, an increase in the lightly silicified diatom Pseudo-nitzschia spp., evidence for more dinoflagellates, and shifts in trophic structure (Rabalais et al., 1996; Turner et al., 1998; Dortch et al., 2001; Parsons et al., 2002).
5.3.2 Secondary Production
There is a variety of evidence that nutrients stimulate secondary production and some- times fishery yields in marine ecosystems (Caddy, 1993). Colijn and van Beusekom (2002) summarized the responses of the North Sea ecosystem to increased nutrient con- centrations and loadings. They found increases in the concentration, production, and changes in species composition for the phytoplankton. There were some indications for an increased biomass of macrozoobenthos. A concomitant increase in higher trophic levels, such as fish and shrimp, was difficult to link directly to the eutrophication process.
Similarly for higher trophic levels, a meta-analysis by Micheli (1999) revealed that the effect of adding nutrients to ecosystems with either two or three trophic levels was to increase the phytoplankton biomass, but not the primary grazers of the phyto- plankton. She also reported that the availability of nitrogen and the primary production rate were strongly correlated to the accumulation of phytoplankton, but not of higher trophic levels. Micheli’s analyses demonstrated a weak coupling between phytoplank- ton, mesoplankton, and zooplankton for closed and manipulated systems. She offered three explanations for these results: more complex interactions among zooplankton than our current understanding, differing proportionality of edible and preferred food with higher phytoplankton production, and advection or losses of nutrients or prey in open marine systems.
If the increased primary production that accompanies nutrient enrichment does not result in increased macroconsumer biomass of higher trophic levels then a higher proportion of the total carbon flow must be shunted to smaller consumers/decomposers or it is buried. The carbon burial rates offshore the Mississippi River increased this century as eutrophication occurred (Eadie et al., 1994), but some of the excess carbon may also have entered the microbial food web throughout the water column.
In cases where eutrophication leads to effects such as loss of seabed vegetation and extensive bottom-water oxygen depletion, there are often negative effects of the increased primary production (Caddy, 1993; Boesch et al., 2001; Rabalais and Turner, 2001). In the deepest bottoms of the Baltic Proper, animals have long been scarce or absent because of low oxygen availability. This area was 20,000 km2until the 1940s
(Jansson and Dahlberg, 1999), but since then, about a third of the Baltic bottom area has intermittent oxygen depletion (Elmgren, 1989). Lowered oxygen concentrations