Although seagrasses and marine macroalgae (macro-autotrophs) play critical ecological roles in reef, lagoon, coastal and open-water ecosystems, their response to ocean acidification (OA) and climate change is not well understood. In this review, we examine marine macro-autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [DIC], carbon dioxide [CO2 ], and lower carbonate [CO3 (2-) ] and pH. We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [CO2 ]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro-autotroph photosynthesis is overwhelmingly C3 (≥ 85%) with most species capable of utilizing HCO3 (-) ; however, most are not saturated at current ocean [DIC]. These results, and the presence of CO2 -only users, lead us to conclude that photosynthetic and growth rates of marine macro-autotrophs are likely to increase under elevated [CO2 ] similar to terrestrial C3 species. In the tropics, many species live close to their thermal limits and will have to up-regulate stress-response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [CO2 ] effects on thermal acclimation are unknown. Fundamental linkages between elevated [CO2 ] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [CO2 ] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H(+) and DIC. These fluxes control micro-environments that promote calcification over dissolution and may be more important than CaCO3 mineralogy in predicting macroalgal responses to OA. Calcareous macroalgae are highly vulnerable to OA, and it is likely that fleshy macroalgae will dominate in a higher CO2 ocean; therefore, it is critical to elucidate the research gaps identified in this review.

Hydrogen sulfide, a phytotoxin that often accumulates in anoxic marine and freshwater marsh soils, suppressed the activity of alcohol dehydrogenase (ADH), the enzyme that catalyzes the terminal step in alcoholic fermentation, in the roots of two wetland macrophytes. This inhibition of root ADH activity with increasing sulfide concentration was associated with decreases in root total adenine nucleotide pool (ATP + ADP + AMP), the adenylate energy charge ratio (AEC), nitrogen uptake (percent recovery of rSNH,+-N) and growth (leaf elongation). These responses were species-specific with a greater negative impact in the freshwater marsh species that naturally inhabits low-sulfide environments. These findings lend support to the hypotheses that ADH activity, as a mcasurc of fermcntative metabolism, is important in maintaining the root energy status of wetland plants under hypoxic-anoxic conditions, that there is a significant negative effect of H,S on the anoxic production of energy in these roots, and that an important negative effect of H,S on plant growth is an inhibition of the energy-dependent process of N uptake.

Anthropogenic nutrient inputs to the northern Everglades of Florida during the last three decades have resulted in alteration of vegetation and soil nutrient storage. Due to the nutrient-limited status of this ecosystem, increased loading may have altered the capacity for long-term nutrient accumulation. Our study was conducted to determine the potential long-term nutrient accumulation rates for this ecosystem along a gradient of nutrient loading. Accumulation rates were calculated using the vertical peat accretion rates, as determined by l37 Cs dating, and nutrient concentration profiles. Intact soil cores were obtained along a 15-km transect and evaluated as a function of distance from the inflow structure. Soil cores were sectioned into 1-cmdepth increments and analyzed for '-"Cs, P, N, C, and selected cations. Vertical accretion rates of peat decreased logarithmically with distance from the inflow, with rates of 1.1 cm yr~' at 0.3 km from the inflow to about 0.25 cm yr' 1 in unimpacted sawgrass (Cladium jamaicense Crantz)-dominated areas. Phosphorus, N, and C accumulation rates in soil and floodwater total P concentrations also showed similar relationships. The P accumulation rates ranged from 0.54 to 1.14 g P m-2 yr~' in cattail (Typha spp.)-dominated areas, and 0.11 to 0.25 g P m-2 yr~' in sawgrass-dominated areas. The C/P and N/P accumulation ratios increased with distance from the inflow, suggesting that a greater proportion of P accumulated in the system, compared with C and N. Similar P retention coefficients were obtained when calculated using either changes in surface water total P concentration, or the long-term P accretion rates. These findings suggest that P was either directly adsorbed by soil or precipitated with Ca in the water column and deposited on the soil surface. This hypothesis was further supported by a highly significant correlation between P and Ca accretion rates, suggesting that Ca-bound P controls equilibrium concentrations in this ecosystem. L ONG-TERM NUTRIENT ACCUMULATION in wetland ecosystems is determined by the balance between inputs and outputs. Nutrients in wetlands undergo several biogeochemical transformations, some resulting in the loss of certain nutrients as gaseous end products or through leaching and discharge to outflow, while

Historically, atmospheric precipitation has been the primary source of N and P to the Florida Everglades. Alterations to the natural hydrology, surface water runoff from agricultural lands, and controlled releases from Lake Okeechobee have increased nutrient loading to the Everglades. A nutrient front encompassing approximately 8000 ha has developed in a northern Everglades marsh, Water Conservation Area 2A (WCA-2A; 44 684 ha), during the last three decades from surface water P and N loading, in addition to atmospheric inputs. Soil cores (0-60 cm) and plant tissue were collected from sawgrass, Cladium jamaicense Crantz, and cattail, Typha domingensis Pers., stands at a distance of 1.6, 5.6, and 9.3 km south of major surface water inflows in WCA-2A: Site N (northern), Site C (central), and Site S (southern), respectively. Although N loading was approximately 10-fold greater at Site N compared with Sites C and S, no significant difference in total N was found between sites at any soil depth. In contrast, P accumulated threefold in soils at Site N compared with Site S (P < 0.05). Organic P accounted for approximately 75% of the total P. Acid-extractable inorganic P (HCI-P,), as an indicator of Ca-bound P, accounted for 80% of the inorganic P and was significantly correlated to dissolved P concentrations of the soil pore water (r = 0.89). Alkali-extractable inorganic P (NaOH-P,), as an indicator of the Feand Al-bound P, comprised 20% of the total inorganic P. High pH values (>8.0) were measured from pore water associated with benthic algal mats. Interstitial P concentrations were 2 to 3 orders of magnitude higher at Site N (>1000 M-g L~') than at Site S (<4 fig L~') and plant tissue N/P ratios at Site N and C were lower, 11:1 compared with 40:1 at Site S. These data suggest P may be an important nutrient limiting primary productivity in the Everglades and that Ca-P precipitation, catalyzed by algal photosynthesis, may be an important mechanism for soil P assimilation.

Increased nutrient loading to the northern Everglades from the nearby Everglades Agricultural Area (EAA) has raised concerns of eutrophication of this oligotrophic wetland. A field study was conducted to determine the influence of nutrient loading on spatial distribution of P, N, C, and related physico‐chemical parameters in the peat soils (Histosols) of Water Conservation Area 2A (WCA‐2A) in the northern Everglades. Field sampling of the top 30 cm of soil was performed at 74 sites across WCA‐2A. Isarithmic plots of N and P forms based on geostatistical analyses revealed widespread enrichment of P, especially in areas proximal to surface inflows importing nutrient‐laden water from the EAA. Enrichment of less magnitude was shown for soil N, while spatial variability of C, bulk density, ash content, and pH were minimal. Concentration of soil porewater NH+4‐N was typically in the 1.5 to 2.5 mg L−1 range in the interior (less impacted) region of WCA‐2A, compared with 4 to 8 mg N L−1 near surface inflows at the northern end of WCA‐2A. In contrast, soluble reactive P in the porewater varied from ≈ 100 µg L−1 or less in the interior marsh to >1000 µg L−1 near inflow structures. Mean soil total P at a depth of 0 to 10 cm was 473 mg kg−1 in the interior marsh, compared with 1338 mg kg−1 in the areas adjacent to inflows. Results of this study show that WCA‐2A soil has served as a net storage for the increased load of P in nutrient‐laden surface inflows. Much of the additional soil P is available for plant uptake, based on the magnitude of P enrichment across a large area. Nutrient enrichment in the soil corresponded with the occurrence of cattails (Typha domingensis Pers. and T. Latifolia L.) in areas previously dominated by sawgrass (Cladium jamaicense Crantz).