Increased macrophyte nitrate reductase activity as a consequence of groundwater input of nitrate through sandy beaches

Abstract

Abstract. While most marine macrophytes preferentially assimilate ammonium to meet growth demand for nitrogen, some also utilize nitrate and exhibit high nitrate reductase activity (NRA). Although nitrate concentrations are often low in coastal waters during the summer and sandy beaches are generally considered to be low nutrient-input habitats, we have observed elevated NRA in leaves of some eelgrass (Zostera marina L.) plants growing immediately adjacent to the shoreline. We postulated that nitrate may become available to eelgrass and macroalgae via groundwater inputs that enter the nearshore water column. To address this possibility, we investigated the availability of groundwater nitrate for the induction of NRA in the leaves of eelgrass and in the macroalgae Sargassum filipendula C. Agardh (Phaeophyceae) and Enteromorpha intestinalis L. Link (Chlorophyceae) collected adjacent to two sandy beaches in the vicinity of Woods Hole, Massachusetts, USA. Induction of NRA was determined in the laboratory for eelgrass collected from one of the beach sites and from an offshore site, Lackey's Bay, which is isolated from groundwater input. At the two beach locations, pore water nitrate concentrations were 100 to 400 #M within a few meters inland from the waterline. Nitrate efflux into the nearshore water column was quite high and variable (2160 ± 660 #tool m-2 h-1) when associated with rapid percolation (37 ___ 11 1 m-2 h-t) of nitrate-enriched pore water. Turbulent wave mixing rapidly diluted the nitrate. Macroalgae and eelgrass growing adjacent to a beach with high nitrate efflux had NR activities three-to sevenfold higher than those of algae and eelgrass growing along a beach section with low nitrate efflux. NRA of eelgrass plants from Lackey's Bay and Great Harbor increased in response to low daily nitrate additions (10 to 25 #M) in the laboratory, with higher nitrate additions (50 to 200/~M) yielding less dramatic responses. The increase in NRA was roughly three times higher for Great Harbor than for Lackey's Bay eelgrass. It appears that * Please address all correspondence and requests for reprints to Dr Pregnall groundwater input of nitrate is sufficient to induce NRA in marine macrophytes growing near some beaches, including those with turbulent wave mixing.

Comparative effects of water-column nitrate enrichment on eelgrass Zostera marina, shoalgrass Halo-dule wrightii, and widgeongrass Ruppia maritime

“…Although septic effluent leachate, sewage and agricultural drainage all can be enriched in ammonium (Valiela et al 1992). a high percentage of the ammonium generally adsorbs to particulate matter and settles out (Simon 1989), or is converted to nitrate as it is transported through oxygenated estuarine waters (Paasche 1988, Maier & Pregnall 1990; but see Lapointe & Clark 1992). Hence, estuaries draining agricultural/urban basins can be dominated by NO3-rather than NH4+, particularly during the critical spring growing season for Zostera marina (Rudek et al 1991, Valiela et al 1992.…”

“…Hence, estuaries draining agricultural/urban basins can be dominated by NO3-rather than NH4+, particularly during the critical spring growing season for Zostera marina (Rudek et al 1991, Valiela et al 1992. Nitrate is especially high relative to phosphorus in septic effluent that leaches to receiving coastal lagoons and upper embayments (Valiela & Costa 1988, Maier & Pregnall 1990. Valiela et al 1990).…”

“…Unless sampling was both well-timed and frequent, major loading events could easily be missed because of removal (e.g. plant uptake and export processes; Maier & Pregnall 1990, Rudek et al 1991. As a supplement to 'instantaneous' assessment of nutrient regime on the basis of concentrations, indices based on eelgrass tissue C, N and P content, or on ratios of these nutrients, should b e developed as parameters that integrate more long-term (days to weeks) fluctuations in nutrient supplies for seagrasses in localized areas where nutrient enrichment is suspected (analogous to use of Thalassia testudinum Banks ex Konig tissue C : N : P content by Fourqurean et al 1993 to infer ecosystem-or regional-scale nutrient availability).…”

“…Within the past few decades, catastrophic losses of seagrass meadows have been correlated with nutrient loading from cultural eutrophication (Borum 1985, Twilley et al 1985, Orth et al 1986). Eutrophication effects on seagrasses are most severe in sheltered habitats with reduced tidal flushing, where nutrient loadings are both concentrated and frequent, and where temperatures fluctuate more widely than in areas with greater water exchange (Stevenson 1988, Maier & Pregnall 1990.…”

The response of primary producer assemblages to mitigation measures to reduce eutrophication in a temperate estuary

“…Eutrophication effects on seagrass meadows are stronger in sheltered environments with frequent and high nutrient loadings, reduced tidal flushing and fluctuating temperatures (Maier and Pregnall, 1990). Increased nitrogen loading has been pointed out as an important cause of seagrass loss by stimulating competition for available light (e.g.…”