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Coastal lagoons are a common land-margin feature worldwide and function as an important filter for nutrients entering from the watershed. The shallow nature of lagoons leads to dominance by benthic autotrophs, which can regulate benthic-pelagic coupling. Here we demonstrate that both microalgae and macroalgae are important in controlling dissolved inorganic as well as organic nitrogen (DIN and DON) fluxes between the sediments and the water column. Fluxes of nitrogen (NH , NO , DON, urea, and dissolved free and combined amino acids [DFAA,DCAA]) and O 2 were measured from October 1998 through August 1999 in sediment cores collected from Hog Island Bay, Virginia. Cores were collected from four sites representing the range of environmental conditions across this shallow lagoon: muddy, high-nutrient and sandy, low-nutrient sites that were both dominated by benthic microalgae, and a mid-lagoon site with fine sands covered by dense macroalgal mats. Sediment-water column DON fluxes were highly variable and comparable in magnitude to DIN fluxes; fluxes of individual compounds (urea, DFAA, DCAA) often proceeded simultaneously in different directions. Where sediment metabolism was net autotrophic because of microalgal activity, TDN (total dissolved nitrogen) fluxes, mostly comprised of DIN, urea, and DFAA, were directed into the sediments. Heterotrophic sediments, including those underlying macroalgal mats, were a net source of TDN, mostly as DIN. Macroalgae intercepted sediment-water column fluxes of DIN, urea, and DFAA, which accounted for 27-75% of calculated N demand. DON uptake was important in satisfying macroalgal N demand seasonally and where DIN concentrations were low. Up to 22% of total N uptake was released to the water column as DCAA. Overall, macroalgae assimilated, transformed, and rereleased to the water column both organic and inorganic N on short (minutes-hours) and long (months) time scales. Microalgae and macroalgae clearly regulate benthic-pelagic coupling and thereby influence transformations and retention of N moving across the land-sea interface.

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Excess nutrient loading and large-scale invasion by nonnatives are two of the most pervasive and damaging threats to the biotic and economic integrity of our estuaries. Individually, these are potent forces, but it is important to consider their interactive impacts as well. In this study we investigated the potential limitation of a nonnative intertidal grass, Spartina alterniflora, by nitrogen (N) in estuaries of the western United States. Nitrogen fertilization experiments were conducted in three mud-flat habitats invaded by S. alterniflora in Willapa Bay, Washington, USA, that differed in sediment N. We carried out parallel experiments in San Francisco Bay, California, USA, in three habitats invaded by hybrid Spartina (S. alterniflora x S. foliosa), in previously unvegetated mud flat, and in native S. foliosa or Salicornia virginica marshes. We found similar aboveground biomass and growth rates between habitats and estuaries, but end-of-season belowground biomass was nearly five times greater in San Francisco Bay than in Willapa Bay. In Willapa Bay, aboveground biomass was significantly correlated with sediment N content. Addition of N significantly increased aboveground biomass, stem density, and the rate of spread into uninvaded habitat (as new stems per day) in virtually all habitats in both estuaries. Belowground biomass increased in Willapa Bay only, suggesting that belowground biomass is not N limited in San Francisco Bay due to species differences, N availability, or a latitudinal difference in the response of Spartina to N additions. The relative impact of added N was greater in Willapa Bay, the estuary with lower N inputs from the watershed, than in San Francisco Bay, a highly eutrophic estuary. Nitrogen fertilization also altered the competitive interaction between hybrid Spartina and Salicornia virginica in San Francisco Bay by increasing the density and biomass of the invader and decreasing the density of the native. There was no significant effect of N on the native, Spartina foliosa. Our results indicate that excess N loading to these ecosystems enhances the vulnerability of intertidal habitats to rapid invasion by nonnative Spartina sp.

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Dissolved organic nitrogen (DON) makes up a large fraction of the total dissolved nitrogen pool in coastal waters, but is often ignored as a potential nitrogen source for primary producers.In laboratory experiments, we measured the uptake of small, labile DON compounds, urea and a variety of different amino acids, by the common estuarine macroalgae Ulva lactuca and Gracilaria vermiculophylla. Urea uptake was measured based on its disappearance from solution; amino acid uptake was measured using this method as well as by assimilation of 15 N-and 13 C-labeled amino acids. The Michaelis-Menten uptake parameters (K m , V max , V max /K m ) were calculated for all compounds. Whereas both species were capable of assimilating urea and amino acids, U. lactuca consistently exhibited significantly higher uptake rates than G. vermiculophylla. There were 2 distinct phases of uptake for urea, an initially rapid 'surge' uptake phase and a slower, 'sustained' phase. This suggests that both species can take advantage of pulsed urea availability. V max rates for urea for U. lactuca were lower than published values for ammonium uptake, but were still high enough to be a significant factor at natural urea concentrations. We did not observe surge uptake of amino acids by either species and the uptake rates varied substantially among the 6 amino acids studied. The differential uptake of 15 N and 13 C by U. lactuca suggested that both alanine and glycine are decarboxylated prior to uptake. However, following decarboxylation, the residual of the glycine molecule is assimilated, while the amine group on alanine is likely removed prior to assimilation. Glycine uptake rates by N-starved or NH 4 + -fertilized macroalgae were higher than uptake rates by NO 3 --fertilized macroalgae, which suggests that the induction of glycine and ammonium uptake may be similar. The low half-saturation constant (K m ) and the high affinity at low concentrations (V max /K m ) that we measured for amino acids suggests that macroalgae can take advantage of the low concentration of amino acids found in estuarine waters. This study shows that macroalgae are capable of utilizing DON and that under conditions of low inorganic nitrogen availability, organic nitrogen may provide a significant portion of the total N demand.KEY WORDS: Marine macroalgae · Organic nitrogen · Nitrogen uptake · Urea · Amino acids · Ulva · Gracilaria Resale or republication not permitted without written consent of the publisher

Summary This article describes how biological ecology models are adapted to analyze the dynamic structure and function of a consumer electronic product “community.” Treating an entire group of interdependent and continually evolving electronic devices as an ecological community provides a basis for more comprehensive analyses of the energy, material, and waste flows associated with household consumption than would be possible using conventional per product approaches. Results show that, similar to a maturing natural community, the average U.S. household electronic product community evolved from a low‐diversity structure dominated by a few products to a highly diverse, evenly distributed community of products between 1990 and 2010. The maturing community of household electronics experienced increased functionality at a community and product level resulting, in part, from introduction of new products, but primarily as a result of increasing ownership of multifunctional products. Multifunctional mobile products are driving increased functionality in a manner similar to a broadly adaptive invasive species, but the community's high functional redundancy, as the result of device convergence, resembles a stable natural community. These results suggest that future strategies to encourage green design, production, and consumption of consumer electronics should focus on minimizing the total number of products owned by maximizing multifunctionality with convergent device design.