NR (µmol NO2− g DW−1 h−1) Eutrophication and Oligotrophication 12 Control 10 1900 h NO3− pulse 0900 h NO3− pulse 361 N N 1000 1300 1600 1900 2200 0100 0400 0700 1000 Time of day (h) Figure 13 The response of the seagrass, Zostera marina, to pulsed water-column NO3À enrichment in light and dark periods Plant NO3À uptake is indicated as leaf activity of the enzyme (nitrate reductase (NR), used to actively take up NO3À) of previously unenriched shoots (means71 standard error) A spike of NO3 À (110 mg NO3 À N lÀ1 ) was added in the morning (white arrow) or, to a subsample of plants from the same population, at night (black arrow) NR activity (plotted as micromoles of nitrite product produced per gram dry weight of plant leaf tissue per hour) indicated that Z marina took up NO3À day or night, whenever a pulse was detected In fact, maximal NR activity was significantly higher when NO3À was added during the dark period Reprinted from Touchette BW and Burkholder JM (2000) Overview of the physiological ecology of carbon metabolism in seagrasses Journal of Experimental Marine Biology and Ecology 250: 169–205 consortium, does not cause a similar effect and, instead, can be mildly stimulatory NO3 À uptake is a metabolically expensive process, requiring high cellular energy Research by Touchette and Burkholder (2000) indicated that sustained water-column NO3 À uptake by Z marina can promote severe internal carbon imbalances, apparently from the need to shunt C skeletons from photosynthesis for use in high amino acid synthesis to prevent internal accumulation of toxic products such as NH3 The physiological mechanism of an internal ‘‘carbon drain’’ from sustained NO3 À uptake was earlier documented for algae by Turpin et al (1991) A common trait of Z marina shoots under excessive water-column NO3 À enrichment is structurally weakened growing regions, perhaps analogous to the abovementioned loss of stem strength that has been reported in certain freshwater emergent macrophytes under NO3 À enrichment Excessive Ni enrichment has also promoted seagrass attack by pathogens (e.g., the slime mold Labrynthula zosteroides), hypothesized to occur because N and C are internally shunted to amino acid production rather than to production of alkyloids and other antimicrobial compounds Another seagrass that has been examined for the NO3 À inhibition phenomenon, Halodule wrightii, and certain macroalgae (e.g., Ulva lactuca) have shown depressed growth in response to NO3 À enrichment, although at much higher N levels (ca 1.4 mg NO3 À N lÀ1 , pulsed daily for 4–5 weeks) than for Z marina (50–110 mg NO3 À N lÀ1 , pulsed daily for 5–8 weeks) Ruppia maritima has been experimentally stimulated by high water-column NO3 À but inhibited by elevated Ni as NH4 ỵ ; and Z marina has been experimentally inhibited by high NH4 ỵ levels, as well In mesocosm experiments, light reduction has been shown to exacerbate the inhibitory effects of water-column NO3 À enrichment on shoot production in Z marina Warm temperatures exacerbate water-column NO3 À enrichment impacts on root growth of Z marina, as well, suggesting that warming trends in climate change may be expected to interact with eutrophication to adversely affect this beneficial habitat species Invasive macrophytes such as Hydrilla verticillata (hydrilla) or Egeria densa (Brazilian elodea) are also a concern in ecosystems that have been altered by nutrient loading These plants appear to be most common in systems that have received excessive N loading relative to P, or systems in which P has been removed but N loads remain high Examples of ecosystems invaded by these plants after P removal include the Potomac River, a tributary of Chesapeake Bay, and the San Francisco Bay Delta Both systems receive (or historically received) high nutrient loads from sewage discharge, but have sustained reduced P loading due to improved effluent treatment and removal of P from laundry detergents Highly productive invasive plants like H verticillata and E densa, have been characterized by Yarrow et al (2009) as ‘‘ecological engineers.’’ They can thrive in turbid waters but also tend to trap sediments, thereby reducing turbidity They also provide habitat for zooplankton and fish, and through their productivity, they alter pH and both water-column and sediment nutrient chemistry Microfauna Freshwater Communities Whereas plants and mixotrophic algae respond directly to nutrient enrichment, animals are generally indirect recipients