Sawgrass, Cladium jamaicense, is the dominant macrophyte in the Florida Everglades. We examined sawgrass flowering phenology and compatibility reactions in ex situ and in situ populations over 2 yr. Sawgrass flowers in May in southern Florida. Flower maturation was relatively synchronous within an inflorescence. Along the entire inflorescence, functionally male flowers emerged initially, followed by stigmas, then anthers of hermaphroditic flowers. Flowers of each sex expanded over 2 d with less than 1 d in between, totaling 6- 7 d for an inflorescence to complete flowering. Hand pollinations showed that sawgrass was self-compatible and not pollen-limited, because open pollinations produced fruit set similar to self- and cross-pollinations. Fruit set was low in autogamy and manipulation treatments. Manipulation treatments were used to study the effect of exposure to airborne pollen during hand pollinations. This treatment thus provides a useful technique for studies on the in situ compatibility of wind-pollinated graminoids. Sawgrass was able to self-fertilize, but the timing of flower maturation on an inflorescence promoted outcrossing. Actual outcrossing rates in sawgrass thus depend on clonal architecture and the timing of floral maturation on other inflorescences within a clone rather than on inflorescences of other genets in a population.

Two strategies for phosphorus (P) economy in P-limiting environment are conservation of use and enhanced acquisition. Using two wetland macrophytes as an example, we show how these strategies change when the P-limitation is removed. Phosphorus resorption and activities of root phosphatases were evaluated over 4 years in Eleocharis cellulosa Torr. and Typha domingensis Pers. from nutrient addition experiment (P, N, N&P, control) established in 15 P limited marshes of Belize. We hypothesized that after P addition both species will increase tissue P content and decrease P resorption efficiency and root phosphatase activity. Initially high phosphorus resorption efficiency, PRE, significantly decreased in Eleocharis 2 years after the first nutrient addition, while no significant decrease was recorded for Typha. Even more dramatic was 5-to 6-fold increase in P in senescent tissues of Eleocharis as compared to less than 2-fold increase in Typha. Root phosphatase activity was high for both species from control plots. After P addition, Eleocharis showed 35% to 70% decrease in enzyme activity correlated to availability of inorganic P in sediments. Eleocharis and Typha employ the "conservation of use" strategy when growing in P limited oligotrophic marshes. In addition, Eleocharis is also using the "enhanced acquisition" strategy. These strategies change when the P limitation is removed but the response varies between the two species and thus changes in the proportion of these two species in a community may result in differences in ecosystem processes such as decomposition.

Different functional groups of macrophytes vary in their impact on aquatic ecosystem structure and processes. The introduction of new species with different growth form, combined with a stochastic event, may have serious and irreversible consequences on lake functioning. Our goals were to document and explain physical, chemical, metabolic and biotic changes in the littoral zones of a volcanic lake before and following two coinciding events: invasion by a submersed macrophyte, Hydrilla verticillata (Hydrocharitaceae), followed by a rapid increase in the lake water level (>2.5 m). We recorded plant biomass, plant tissue C:N:P stoichiometry, macroinvertebrates, water characteristics data along transects through littoral zones, and measured gas emission in controlled mesocosms and in the lake. The native emergent species, Schoenoplectus californicus (Cyperaceae), was generally not able to survive such a rapid water level increase, and Hydrilla spread and formed dense mats further preventing Schoenoplectus regeneration. The impact of another introduced species, the free‐floating Eichhornia crassipes (Pontederiaceae), was more localised, despite its much longer presence at the lake. Although the three species had comparable standing biomass, the two invader species had lower C:N:P ratios than Schoenoplectus, resulting in faster decomposition rates and indicating potential shifts in nutrient cycling within the ecosystem. The oxygen profile of the water column was altered by the non‐native species in a significantly different manner: in Eichhornia, the saturation concentrations dropped down to 30%–50% of dissolved oxygen, while oxygen supersaturation was recorded in Hydrilla. Both Schoenoplectus and Eichhornia patches exhibited comparable carbon dioxide (CO2) fluxes, sequestering 230 and 300 mg CO2 m−2 hr−1, respectively, during the day and emitting 250 and 200 mg CO2 m−2 hr−1, respectively, during the night. Contrary to these two species, Hydrilla patches sequestered CO2 during the day (34 mg CO2 m−2 hr−1) and night (44 mg CO2 m−2 hr−1). The invasive species maintained a richer community of macroinvertebrates compared to several native species (excluding Schoenoplectus), both in taxa diversity and in numbers of individuals. When the results are considered in the regional context, an increase in nutrient supply could lead to the dominance of free‐floating plants. We discussed management options more broadly considering the negative impacts of introduced species balanced against their beneficial effects, in the context of environmental changes.

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Oligotrophic, phosphorus (P) limited herbaceous wetlands of northern Belize are being impacted by P loading from fertilizer runoff. P enrichment causes a shift in autotroph communities from a microphyte (cyanobacterial mats, CBM) to macrophyte (Eleocharis spp., Typha domingensis) dominated system. To document potential effects of P, salinity, and macrophyte species on the heterotrophic microbial community nutritional status (represented especially by specific phospholipids fatty acids and specific respiration rate), biomass and activities, we took soil samples from established P enrichment plots in replicated marshes of two salinity levels. P addition increased microbial biomass carbon (C), nitrogen (N) and P, as well as soil nutrient transformation rates (nitrogenase activity, N mineralization and immobilization, methanogenesis). The effect of plant species (Eleocharis vs Typha sites) was generally lower than the effect of P addition (CBM vs Eleocharis sites) and was most evident at the low salinity sites, where Eleocharis dominated plots had enhanced nitrogenase activity and P microbial immobilization. Salinity reduced the overall rates of microbial processes; it also weakened the positive effect of both P addition and plant species on microbial activities. Lastly, the amount of N stored in microbial cells, likely in form of osmoprotective compounds, was enhanced by salinity.

In order to determine the impact of nutrient enrichment on phosphorus (P) limited wetlands, we established experimental P additions in marshes throughout northern Belize. P significantly increased macrophyte primary production, which led to the rapid elimination of cyanobacterial mats. The replacement of cyanobacterial mats by macrophytes constrained autotrophic nitrogen (N) fixation, increased the quantity, and changed the quality of organic matter input to the sediments. We predicted that the activity of sediment heterotrophic N fixers will be impacted by these alterations in carbon input. We used the acetylene reduction technique to measure potential (glucose amended) nitrogenase activity (NA) in sediments from controls and treatment plots that have been P enriched for four years and dominated either by Eleocharis cellulosa, or Typha domingensis for two years. NA in P-enriched plots was 2-3 orders of magnitude higher than NA in controls. NA was positively correlated with the soil reactive P, both total organic and microbial carbon, live root biomass, and total phospholipid fatty acids (PLFA) as an indicator of active microbial biomass. It was negatively correlated with the concentration of ammonium-N. Path analysis revealed that the indirect effect of P on NA through the root biomass was more important than the direct effect of P. NA of the upper sediment layer was consistently higher in Eleocharis than in Typha dominated plots, despite the higher litter input by Typha. We feel that the higher levels of lignin and phenolics occurring in Typha litter, relative to Eleocharis, constrained NA in Typha plots.

We used established long-term experimental P-amended plots in freshwater marshes of northern Belize to determine the impact of P input on nitrogen (N) fixation. Marshes with different conductivities and sulfate concentrations were selected to elucidate the effect of salinity and the contribution of sulfur reducing bacteria to the overall N fixation. Rates of N fixation in sediment, roots, and cyanobacterial mats was measured in laboratory incubation experiments (acetylene reduction assay calibrated by 15 N 2 reduction assay) with and without the addition of sodium molybdate (sulfur reducing bacteria inhibitor). P has increased macrophyte primary production significantly, which led to the rapid elimination of cyanobacterial mats and the elimination of autotrophic N fixation. P addition enhanced heterotrophic N fixation in both the sediments and rhizosphere due primarily to increased C supply to the sediment. When expressed on a dry weight basis, root associated N fixation was higher than sediment N fixation, but the contribution of the root associated fixation to the total N fixation was small when expressed per square meter. Sulfur reducing bacteria were an important component of N fixation, contributing from 20 to 53% to the overall N fixation. A simple N budget was created to determine if N demands are met following P addition. The heterotrophic N fixation substituted in part for autotrophic cyanobacterial N fixation when P limitation was alleviated.