Short-term changes in phosphorus storage in an oligotrophic Everglades wetland ecosystem receiving experimental nutrient enrichment

Natural, unenriched Everglades wetlands are known to be limited by phosphorus (P) and responsive to P enrichment. However, whole-ecosystem evaluations of experimental P additions are rare in Everglades or other wetlands. We tested the response of the Everglades wetland ecosystem to continuous, low-level additions of P (0, 5, 15, and 30 μg L−1 above ambient) in replicate, 100 m flow-through flumes located in unenriched Everglades National Park. After the first six months of dosing, the concentration and standing stock of phosphorus increased in the surface water, periphyton, and flocculent detrital layer, but not in the soil or macrophytes. Of the ecosystem components measured, total P concentration increased the most in the floating periphyton mat (30 μg L−1: mean = 1916 μg P g−1, control: mean = 149 μg P g−1), while the flocculent detrital layer stored most of the accumulated P (30 μg L−1: mean = 1.732 g P m−2, control: mean = 0.769 g P m−2). Significant short-term responses of P concentration and standing stock were observed primarily in the high dose (30 μg L−1 above ambient) treatment. In addition, the biomass and estimated P standing stock of aquatic consumers increased in the 30 and 5 μg L−1 treatments. Alterations in P concentration and standing stock occurred only at the upstream ends of the flumes nearest to the point source of added nutrient. The total amount of P stored by the ecosystem within the flume increased with P dosing, although the ecosystem in the flumes retained only a small proportion of the P added over the first six months. These results indicate that oligotrophic Everglades wetlands respond rapidly to short-term, low-level P enrichment, and the initial response is most noticeable in the periphyton and flocculent detrital layer.

Ecosystem Structure and Function Still Altered Two Decades After Short-Term Fertilization of a Seagrass Meadow

An oligotrophic phosphorus (P) limited seagrass ecosystem in Florida Bay was experimentally fertilized in a unique way. Perches were installed to encourage seabirds to roost and deliver an external source of nutrients via defecation. Two treatments were examined: (1) a chronic 23-year fertilization and (2) an earlier 28-month fertilization that was discontinued when the chronic treatment was initiated. Because of the low mobility of P in carbonate sediments, we hypothesized long-term changes to ecosystem structure and function in both treatments. Structural changes in the chronic treatment included a shift in the dominant seagrass species from Thalassia testudinum to Halodule wrightii, large increases in epiphytic biomass and sediment chlorophyll-a, and a decline in species richness. Functional changes included increased benthic metabolism and quantum efficiency. Initial changes in the 28-month fertilization were similar, but after 23 years of nutrient depuration T. testudinum has reestablished itself as the dominant species. However, P remains elevated in the sediment and H. wrightii has maintained a presence. Functionally the discontinued treatment remains altered. Biomass exceeds that in the chronic treatment and indices of productivity, elevated relative to control, are not different from the chronic fertilization. Cessation of nutrient loading has resulted in a superficial return to the pre-disturbance character of the community, but due to the nature of P cycles functional changes persist.

Short-term changes in phosphorus storage in an oligotrophic Everglades wetland ecosystem receiving experimental nutrient enrichment

Natural, unenriched Evergladeswetlands are known to be limited by phosphorus(P) and responsive to P enrichment. However,whole-ecosystem evaluations of experimental Padditions are rare in Everglades or otherwetlands. We tested the response of theEverglades wetland ecosystem to continuous,low-level additions of P (0, 5, 15, and30 μg L−1 above ambient) in replicate,100 m flow-through flumes located in unenrichedEverglades National Park. After the first sixmonths of dosing, the concentration andstanding stock of phosphorus increased in thesurface water, periphyton, and flocculentdetrital layer, but not in the soil or macrophytes. Of the ecosystem components measured, total P concentration increased the most in the floating periphyton mat (30 μg L−1: mean = 1916 μg P g−1, control: mean =149 μg P g−1), while the flocculentdetrital layer stored most of the accumulated P(30 μg L−1: mean = 1.732 g P m−2,control: mean = 0.769 g P m−2). Significant short-term responsesof P concentration and standing stock wereobserved primarily in the high dose (30 μgL−1 above ambient) treatment. Inaddition, the biomass and estimated P standingstock of aquatic consumers increased in the 30and 5 μg L−1 treatments. Alterationsin P concentration and standing stock occurredonly at the upstream ends of the flumes nearestto the point source of added nutrient. Thetotal amount of P stored by the ecosystemwithin the flume increased with P dosing,although the ecosystem in the flumes retainedonly a small proportion of the P added over thefirst six months. These results indicate thatoligotrophic Everglades wetlands respondrapidly to short-term, low-level P enrichment,and the initial response is most noticeable inthe periphyton and flocculent detrital layer.

Arctic ecosystem responses to changes in water availability and warming: Short and long-term responses

Arctic soils store close to 14% of the global soil carbon. Most of arctic carbon is stored below ground in the permafrost. With climate warming the decomposition of the soil carbon could represent a significant positive feedback to global greenhouse warming. Recent evidence has shown that the temperature of the Arctic is already increasing, and this change is associated mostly with anthropogenic activities. Warmer soils will contribute to permafrost degradation and accelerate organic matter decay and thus increase the flux of carbon dioxide and methane into the atmosphere. Temperature and water availability are also important drivers of ecosystem performance, but effects can be complex and in opposition. Temperature and moisture changes can affect ecosystem respiration (ER) and gross primary productivity (GPP) independently; an increase in the net ecosystem exchange can be a result of either a decrease in ER or an increase in GPP. Therefore, understanding the effects of changes in ecosystem water and temperature on the carbon flux components becomes key to predicting the responses of the Arctic to climate change. The overall goal of this work was to determine the response of arctic systems to simulated climate change scenarios with simultaneous changes in temperature and moisture. A temperature and hydrological manipulation in a naturally-drained lakebed was used to assess the short-term effect of changes in water and temperature on the carbon cycle. Also, as part of International Tundra Experiment Network (ITEX), I determined the long-term effect of warming on the carbon cycle in a natural hydrological gradient established in the mid 90's. I found that the carbon balance is highly sensitive to short-term changes in water table and warming. However, over longer time periods, hydrological and temperature changed soil biophysical properties, nutrient cycles, and other ecosystem structural and functional components that down regulated GPP and ER, especially in wet areas.