Eutrophication and Oligotrophication pressure on the phytoplankton The increased anoxic/hypoxic conditions from yet-higher phytoplankton production cause further reductions in benthic invertebrate species and abundance, leading to more intense cyprinid grazing pressure on zooplankton Thus, highly eutrophic lakes contain dense phytoplankton and food-limited (stunted) cyprinids Emergent macrophytes at the littoral fringe of lakes, rivers, estuaries, and marine coasts can maintain enhanced growth and biomass over a wide range of nutrient inputs Under certain conditions with sustained high nutrient loading, however, emergent macrophytes can decline The nutrients stimulate increased growth, greater stem density and, often, accelerated longitudinal growth, leading to higher intraspecific competition for light and weakening of stems The elevated nutrients also promote higher suspended algal growth as well as higher growth of epiphytes on the submersed portions of the macrophyte stems The increased weight makes the plants less well anchored, leading to dieback at the water’s edge Elevated Ni also promotes reduction in supporting vascular tissues, and loss of stem strength Estuarine and Marine Communities Brackish waters are colonized by rooted, mostly freshwater species with moderate salt tolerance (as examples, certain Potamogeton spp., Valisneria americana, Zanichellia) In such habitats, light is often the primary resource limiting growth Species with broader salt tolerance such as Ruppia maritima also can be abundant Only about 50 species of angiosperms, mostly close relatives of freshwater Potamogeton spp., have the salt tolerance needed to thrive in marine habitats Nearly all of these seagrasses grow in muddy substrata of shallow coastal lagoons and quiet embayments Estuarine and marine macrophytes tend to be highly sensitive to light reduction and, thus, susceptible to eutrophication-related turbidity from algal (phytoplankton, epiphyte, and macroalgae) overgrowth and sediment loading/resuspension Such shading causes gradual dieback and loss of most SAV, leading to dramatic declines, in turn, in the diversity and abundance of many species that depend on the habitat provided by these plants More sensitive SAV species are replaced by others that are tolerant to eutrophic conditions – for example, among subtropical seagrasses, field observations and limited experiments have indicated that turtle grass (Thalassia testudinum) is more sensitive to eutrophication than shoal grass (Halodule wrightii) and manatee grass (Syringodium filiforme) Other regions may not have additional seagrass species available to replace more sensitive species and, even where such species are present, they typically offer less desirable fish nursery habitat than the former dominant Thus, in seagrass meadows under nutrient over-enrichment, more oligotrophic seagrass species are replaced by less sensitive species when available As eutrophication progresses the seagrasses are eliminated, and rapidly growing macroalgae and phytoplankton become the dominant flora Although light reduction is considered the major mechanism for seagrass decline under cultural eutrophication, excessive nutrients can act independently of light to promote seagrass loss (Figure 12) The dominant north temperate species, eelgrass (Zostera marina), apparently lacks a physiological mechanism to inhibit NO3 À uptake through its leaves, Light reduction Shoot number (% of control) 360 30% Io 50% Io 70% Io –20 –40 –60 Low N High N Figure 12 The effects of water-column NO3 À enrichment and light reduction on shoot production of the seagrass, Zostera marina From the first author’s outdoor mesocosm experiments, indicated as the percent decrease from shoot production of control plants that did not receive water-column NO3 À additions or light reduction (except that plants in controls and treatments all received an additional 30% light reduction for h at 09.00, 12.00, and 15.00 h on a 3-day rotation using neutral density screens to stimulate conditions during high tide) Treatments were imposed for 10 weeks during the fall growing season for Z marina Controls were maintained at ambient natural light (except during simulated high tide) and NO3 À (o30 mg NO3 À N lÀ1 ) Treatments included low N (at 50 mg NO3 À N lÀ1 , or 3.5 mM NO3 ÀN) and high N (at 100 mg NO3 À N lÀ1 , or mM NO3 À N; low and high NO3 À additions were added in the morning as a pulse of enrichment) at each of three imposed light levels as 30, 50, or 70% reduction of ambient surface light (Io, accomplished using the neutral density screens, with additional shading to simulate high tide as noted) Water exchange (5–10% of the tank volumes per day replaced with fresh estuarine water) was done in late afternoons; see Burkholder et al (1992), for further details about the mesocosm system In all treatments with water-column NO3 À enrichment, Z marina declined in shoot production relative to shoot production of control plants, and the NO3 À inhibition effect was exacerbated by light reduction (means ỵ standard error; Po0.05, n ¼ mesocosms for controls and for each treatment) These effects were not caused by algal overgrowth, which was maintained at low levels in controls and all treatments throughout the experiment as indicated by the research of Roth and Pregnall (1988) and Burkholder et al (1992) Most plants take up NO3 À during the day with the energy from photosynthesis In contrast, Z marina takes up water-column NO3 À day or night if it becomes available, as shown by Touchette and Burkholder (2000) (Figure 13) This species is thought to have evolved in N-poor coastal waters, and sustained NO3 À uptake under temporary enrichment may have developed as a once highly advantageous competitive mechanism or ‘‘strategy’’ However, as coastal waters have become more eutrophic from sewage, septic effluent leachate, agricultural sources, and other anthropogenic sources, sustained uptake of water-column NO3 À likely has become a disadvantage NO3 À enrichment of the sediments, under control by an abundant microbial