Salt marshes link terrestrial and aquatic ecosystems provide numerous ecosystem services such as functioning as a critical habitat for fish and wildlife and nutrient and pollutant removal. My current research in salt marshes across Mobile Bay, Alabama primarily aims to understand (1) how ecosystem function changes along a salinity gradient within salt marsh habitats and (2) how does nutrient removal vs retention change in salt marshes protected by engineered breakwaters, designed to decrease wave height and protect shorelines from erosion, relative to control marshes without breakwaters. Additionally, I am working with engineers and social scientists to evaluate how salt marshes will persist with sea-level rise, how salt marshes can protect infrastructure from flooding, and what types of nature-based solutions are of interest to stakeholders around Mobile Bay to protect infrastructure from flooding.
Leaf litter decomposition is an essential ecosystem function whereby plant material is broken down into its elements. Decomposition is mediated by microbes (bacteria and fungi) and macroinvertebrates. In freshwater ecosystems, macroinvertebrates are known to be important in the decomposition process and can accelerate decomposition rates. However, less is known about the role of invertebrates in plant decomposition in brackish and salt marshes as decomposition studies in salt marshes often use small mesh sizes which exclude invertebrates. To address this knowledge gap, I conducted a decomposition experiment spanning a salinity gradient in Mobile Bay using litter bags with fine mesh ( < 0.5 mm opening) to exclude invertebrates and coarse mesh (5 mm mesh opening) to allow invertebrates to freely colonize. I am finding that the effect of litter bag mesh size (fine vs coarse) on plant decomposition rates depends on the specific marsh (freshwater, brackish, saline).
Leaf litter bags deployed in a salt marsh to measure the relative influence of aquatic macroinvertebrates on marsh plant decomposition rates. Fine mesh litter bags (<0.5 mm opening) are black and coarse mesh litter bags (5 mm opening) are red.
In coastal environments, nature-based solutions like breakwaters are increasingly used to protect shorelines from erosion and promote marsh resilience to sea-level rise. While breakwaters are designed to decrease wave height and energy, they may also alter patterns of sediment deposition, marsh plant distribution, and carbon storage, with potential consequences for other ecosystem services. I am investigating how breakwaters impact nitrogen removal (denitrification) and retention (dissimilatory nitrate reduction to ammonium; DNRA) in Bon Secour Bay, Alabama. We are measuring potential denitrification and DNRA rates using Membrane Inlet Mass Spectrometry at the University of Alabama. We had a successful summer field sampling campaign and look forward to measuring these rates again in the winter.
A breakwater structure made of stone, designed to decrease wave height and protect the Swift Tract shoreline from erosion (left). The salt marsh along the Swift Tract shoreline where I am measuring potential denitrification rates (right).
Manatee effects on freshwater ecosystems
Resource subsidies are energy and nutrients that are created in a donor habitat that are transported to a recipient habitat where they are used and have a substantial impact on the organisms living in the recipient ecosystem. For example, Pacific salmon migrating from the ocean to freshwater represents a subsidy because they are transporting marine nutrients to the freshwater ecosystem via their carcasses when they die and through their excretions. My PhD research focused on resource subsidies.
The Florida manatee primarily lives in marine ecosystems, but migrates into Florida’s warm, spring-fed ecosystems in the winter such as in Kings Bay. The springs function as thermal refugia for manatees when the ocean temperatures drop below 20 degrees Celsius because unlike the oceans, springs maintain constant temperature of 22 degrees year-round. What are the ecosystem consequences of these 1,000 lb animals migrating into the springs in the hundreds? My dissertation research attempted to answer this question through a number a different studies by focusing on the potential role of manatees providing resource subsidies through their feces and urine, both of which are rich in nitrogen and phosphorus. I measured how manatee feces functioned as a detritial food resource for aquatic macroinvertebrates and found that manatee feces increased macroinvertebrate production. We measured if manatees provide nutrients to freshwater biofilms (algae, fungi, organic matter) and how manatee presence/absence affected biofilms. We also studied how manatees affected nutrient uptake in the sediment and the water column through their excretion of urea and bioturbation (mixing) of the sediment. Urea introduced by manatees may may alter uptake of ammonium, nitrate, and phosphate. Bioturbation of sediments may also alter nutrient uptake rates by releasing nutrients buried in the sand but also decrease light availability for biofilms.
Potential mechanisms by which manatees may affect spring-fed ecosystems.
The effects of drought on aquatic insect emergence
Drought is becoming more common with climate change. The effects of drought on streams can be severe, resulting in increased water temperatures, loss of connectivity with the riparian zone, and in the most extreme cases, loss of longitudinal connectivity of the stream network. Loss of longitudinal connectivity can result in pool habitats being the last remaining refugia for aquatic life. We measured how an extreme drought in 2018, which resulted in the loss of longitudinal connectivity of the stream network leaving only isolated pools, affected aquatic insect emergence on the Konza Prairie Biological Station. We returned to the same pools in 2020 and resampled emergence under non-drought conditions when stream flow resumed. We found decreased emergence abundance during the drought conditions relative to the non-drought sampling conditions, but no differences in emergence biomass between the two sampling periods. We found emerging midges, which accounted for 80% of the emergence biomass, were approximately 50% larger during the drought relative to the non-drought conditions and offset the decreased emergence abundance. The larger midge body size during the drought was unexpected as metabolic theory predicts organism body size should decrease under increased temperatures.
How leaf identity affects the transfer of energy up the food web
Leaf litter (dead leaves) can be the dominant source of energy in headwater streams because tree canopies can shade streams limiting algal photosynthesis. Leaves that decompose rapidly are often thought to be a higher quality resource for aquatic organisms than leaves that decompose slowly. Rapidly decomposing leaves often have high concentrations of nitrogen and phosphorus, which can be limiting nutrients for invertebrates, and low concentrations of lignin, tannins, and phenols, which decrease leaf decomposition rates. However, rapidly decomposing leaves disappear quickly, before invertebrates can use them as a food resources, whereas slowly decomposing leaves will persist in a stream for a longer period of time.
My master's research used leaves enriched with 13C and 15N stable isotopes to measure the transfer of carbon and nitrogen from leaves to invertebrates feeding on the leaves. We found that invertebrates feeding on slowly decomposing leaves incorporated more of the leaf mass lost during decomposition than invertebrates feeding on rapidly decomposing leaves. Our results demonstrate the importance of slowly decomposing litter as it will provide sustained energy and nutrients to food webs over a longer period of time than rapidly decomposing litter. Rapidly decomposing litter, however, can provide a quick pulse of energy to food webs and likely loses more mass to the microbial pathway than slowly decomposing litter.