Anthropogenic nutrient enrichment of seagrass and coral reef communities in the Lower Florida Keys: discrimination of local versus regional nitrogen sources

摘要

Land-based nutrient pollution represents a significant human threat to coral reefs globally. We examined this phenomenon in shallow seagrass and coral reef communities between the Content Keys (southern Florida Bay) and Looe Key (south of Big Pine Key) in the Lower Florida Keys by quantifying the role of physical forcing (rainfall, wind, tides) and water management on mainland South Florida to nutrient enrichment and blooms of phytoplankton, macroalgae, and seagrass epiphytes. Initial studies (Phase Ⅰ) in 1996 involved daily water quality sampling (prior to, during, and following physical forcing events) at three stations (AJ, an inshore area directly impacted by sewage discharges; PR, a nearshore patch reef located inshore of Hawk Channel; and LK, an offshore bank reef at Looe Key) to assess the spatial and temporal patterns in advection of land-based nutrients to the offshore reefs. Concentrations of dissolved inorganic nitrogen (DIN=NH_4~++NO_3~- + NO_2~-), soluble reactive phosphorus (SRP), and chlorophyll a increased at PR and LK following a wind event ( ～ 15 knots, northeast) in mid-February. The highest DIN (mostly NH_4~+) and SRP concentrations of the entire study occurred at the inshore AJ during an extreme low tide in March. Following the onset of the wet season in May, mean NH_4~+ and chlorophyll a concentrations increased significantly to maximum seasonal values at PR and LK during summer; relatively low concentrations of NO_3~- and a low f-ratio (NO_3~-/NH_4~+ + NO_3~-) at all stations during summer do not support the hypothesis that the seasonal phytoplankton blooms resulted from upwelling of NO_3~-. A bloom of the seagrass epiphyte Cladosiphon occidentalis (phaeophyta) followed the onset of the rainy season and increased NH_4~+ concentrations at LK, resulting in very high epiphyte:blade ratios ( ～ 3:1) on Thalassia testudinum. Biomass of macroalgae increased at all three stations from relatively low values (< 50 g dry wt m~(-2)) in winter and early spring to higher values ( ～ 100-300 g dry wt m~(-2)) typical of eutrophic seagrass meadows and coral reefs following the onset of the rainy season. The mean δ~(15)N value of Laurencia intricata (rhodophyta) during 1996 at AJ ( + 4.7 per thousand) was within the range reported for macroalgae growing on sewage nitrogen; lower values at the more offshore PR (+ 3. 1 per thousand) and LK ( + 2.9 per thousand) were at the low end of the sewage range, indicating an offshore dilution of the sewage signal during the 1996 study. However, transient increases in δ~(15)N of Cladophora catanata (chlorophtyta) from ～+2% to + 5% at LK concurrent with elevated NH_4~+ concentrations following rain and/or wind events in May and July suggest episodic advection of sewage nitrogen to the offshore LK station. The Phase Ⅱ study involved sampling of macroalgae for (δ~(15)N along a gradient from the Content Keys through Big Pine Key and offshore to LK in the summer wet season of 2000 and again in the drought of spring 2001. During the July 2000 sampling, macroalgae in nearshore waters around Big Pine Key had elevated δ~(15)N values (～+4 per thousand ) characteristic of sewage enrichment; lower values (～ + 2 per thousand) at LK were similar to values reported for macroalgae in upstream waters of western Florida Bay influenced by nitrogen-rich Everglades runoff.