Introducing Jessica Balerna, Ph.D. (and a new publication too!)
That's right, everyone. After 6 long years, I successfully defended my dissertation on March 7, 2023.
And I know you're all dying to know: WHAT DID I FIND???
You can watch a recording of my dissertation presentation here. Otherwise, keep reading for my best attempt at a summary!
As I discuss in my dissertation background page, my dissertation work is all about better understanding feedbacks between management decision-making and wetland condition. More specifically, how shifting water conservation policies—primarily related to cutbacks in groundwater extraction—have changed the ecosystem services wetlands provide including biophysical services, like carbon storage and plant biodiversity, as well as cultural services, like aesthetic value and recreational access.
The first chapter of my dissertation introduces you to the hundreds of freshwater depressional wetlands located in the Tampa Bay region (Fig. 1) and how they fit within a socio-ecological system that creates these important feedbacks.
Fig 1. In this graphic, I show a subset of the hundreds of freshwater wetlands scattered across the Tampa Bay region. 141 of the wetlands shown are located in and around wellfield-parks, or designated areas of groundwater extraction that are also publicly-accessible parks with hiking and biking trails. Another 11 wetlands are located in the Green Swamp Wilderness Preserve (or Green Swamp for short), which is over 25 miles away from the nearest groundwater extraction site and a well known site for hiking .
The second chapter of my dissertation has recently been published in the Journal of Environmental Management and assesses how wetland inundation varies as a function of social and environmental variables. I focus on wetland inundation (which is a catch-all term for how wet a wetland is) because how often and how much a wetland is inundated influences all of those other ecosystem services I just listed above (carbon storage, biodiversity, aesthetic value). Wetland inundation is also likely to respond the fastest to changes in social (think management decision-making around groundwater extraction and land development) and environmental (think precipitation, dominant vegetation communities within the wetland, and wetland size and shape) variables.
In this chapter, I demonstrate that the two strongest factors influencing wetland inundation are precipitation and groundwater extraction. Increasing precipitation increases wetland inundation but increasing groundwater extraction decreases wetland inundation. I also show that during periods of low rainfall and high pumping, wetland inundation is even LOWER than would be expected if either of those things happened alone. What that means is that during a drought, water managers might extract more groundwater to meet human demands because they can't rely on surface water. This is going to have the worst effects on wetland inundation, though, and alternative methods like trying to reduce demands through water conservation efforts would help better protect freshwater wetlands.
Luckily in the Tampa Bay region, water managers cut back groundwater extraction rates in 2009 to their lowest levels in several decades (Fig. 2). I found this significantly increased wetland inundation and created more hydrological diversity among the 152 wetlands (Fig. 1) I studied. Greater hydrological diversity can increase ecological diversity. This finding thus means that freshwater wetlands are not interchangeable with one another and instead each contribute something different to the entire (wet)landscape.
You can read more about this chapter in the recently published peer-reviewed article here.
Fig 2. In this graphic, I show how groundwater extraction has changed over time. The first panel shows from 1930 through 2018, where you can see a rapid, almost uninterrupted, rise in groundwater extraction between 1930 and 1970. The damage that over-extraction caused became more visible in the early 1990s due to wetlands and lakes drying up. Residents were angry and sued water managers creating what journalists called "The Tampa Bay Water Wars." The courts responded by mandating the creation of a new water utility "Tampa Bay Water," that would be responsible for lowering groundwater extraction rates beginning in 1998. The second panel (which zooms in to 1990 through 2018) shows how those cutbacks occurred in stages. As people didn't suddenly just stop needing water, Tampa Bay Water had to develop alternative supply sources like a surface water reservoir and a desalination plant to compensate for lower groundwater extraction rates. The final stage of cutbacks began in 2009 and is the lowest groundwater extraction rates have been since the 1940s.
Moving to the third chapter. I continue thinking about wetland inundation and how it may or may not be recovering as a response to shifting groundwater extraction policy. I showed in the last chapter that wetlands responded to cutbacks in pumping, but did they recover enough to reach the same inundation conditions as seen in wetlands in the Green Swamp (Fig 1), e.g., wetlands never exposed to groundwater extraction??
I found that over 58 % of wetlands had significantly increasing water levels over the time period where groundwater extraction was being cut back (1998–2018). This indicates that these wetlands were experiencing ongoing recovery to groundwater extraction.
Another 16 % of wetlands were not recovering even with cutbacks in groundwater extraction. 14 % of those wetlands had no change in water levels over the study period AND had water levels consistently below what we saw in reference wetlands. 2 % of those wetlands had decreasing water levels over the study period. These findings might be due to the diversity in groundwater extraction rates throughout the region. While I said that pumping has been cut back regionally, when you zoom in on certain areas of the study region, pumping might be higher in some places than in the past.
These findings might also be because of cascading impacts to these wetlands that resulted from decades of over-extraction of groundwater that occurred before cutbacks began in 1998. These cascading impacts might include soil subsidence and karst collapse. Karst is the common hydrogeologic type in this region, compared to bedrock in other areas of the country. Karst is highly permeable meaning water can freely flow through it. This also means, karst is more susceptible to collapse when those pockets that typically hold water become empty due to things like groundwater extraction.
The final 26 % of wetlands were not impacted by changes in groundwater extraction likely because there were layers of clay (non-permeable) preventing flow between wetlands and the underlying Upper Floridan aquifer where groundwater extraction occurs.
Finally, in chapter four, I looked beyond wetland inundation to think about those other ecosystem services that wetlands provide. To study this, I collected soil cores (see this blog post for a close up of how I did that), identified tree species, and surveyed visitors to the wellfield-parks where these wetlands are. These sampling methods helped me better understand soil carbon storage, plant biodiversity, aesthetic value, recreational access, and more in these freshwater wetlands. I then used Principal Component Analysis to visualize trade-offs among these services with groundwater extraction, where trade-offs indicate one service is enhanced while another is impaired.
For example, I found a trade-off between historic groundwater extraction rates (i.e., pre-cutbacks) and biodiversity. This means that wetlands exposed to higher historic rates of groundwater extraction had the lowest plant biodiversity. I mentioned before that wetland inundation was likely to respond the fastest to changes in groundwater extraction. Even though cutbacks began almost 3 decades ago, plant biodiversity may not have had a chance to recover yet (it takes a long time for plants to grow, remember). In another 3 decades, if we repeated my study in the exact same wetlands, we may no longer see this trade-off.
I also found a trade-off between current groundwater extraction rates (i.e., post-cutbacks) and soil carbon storage. This means that wetlands exposed to the highest current rates of groundwater extraction had the lowest carbon storage. While plant biodiversity may need more time to recover because lower rates are associated with historic groundwater extraction rates, soil carbon storage is responding to current rates. This means that the total regional cutbacks may not be enough. Instead, managers may need to lower regional groundwater extraction rates more or focus on lowering groundwater extraction rates in places throughout the region where it is still high enough to create these trade-offs. Otherwise, these wetlands, which typically store a lot of carbon, won't be as helpful in the fight against climate change.
Finally, I found that people prefer wet, diverse wetlands, which means that aesthetic value was also impaired by historic groundwater extraction rates. However groundwater extraction increased recreational access in these wetlands creating a co-benefit between groundwater extraction and at least one other wetland service. This is likely because groundwater extraction often makes wetlands less inundated, which means they're more accessible to recreate around. When wetlands are too wet, they can overflow into surrounding trails. The addition of boardwalks could help increase access to wet wetlands which people prefer to be around!
Phew. I summarize a lot of these details and what it might mean for managers in one of the final slides of my dissertation presentation (which, as a reminder, you can watch above).
Amazingly, my dissertation only touches on a fraction of the data that I have collected during my time in graduate school, which means lots more analysis and writing in my future (and a lot more blog posts to read in your future)! Stay tuned😊