Of marsh and mangrove: coupled biophysical and anthropogenic drivers of 20th century wetland conversion in Tampa Bay Estuary, Florida (USA)

Abstract

Dense mangrove swamps currently dominate tidal wetlands of the Tampa Bay Estuary System on the central peninsular Gulf Coast of Florida (USA). Late-19th century Coast and Geodetic Survey topographical charts and Government Land Office surveys, however, depict wetland systems dominated by salt marsh—therefore suggesting mangrove dominance as a product of 20th century encroachment. To clarify the primary drivers of ecosystem change, this study integrates sedimentological, paleobotanical, and radiometric analyses of sediment cores collected in 2018 with analyses of aerial photography taken between 1940 and 1997. Results empirically ground truth the wetland conversions inferred through analytical comparisons of historical and modern mapping and establish a high-resolution chronology for coastal environmental change. These results showed that salt marsh and salt prairie habitats persisted within Tampa Bay study areas until the mid-20th century. Mangrove forest rapidly encroached the study areas over a 20 year interval, between 1960 and 1980, immediately following intensive ditching for mosquito control. The findings demonstrate how coastal geoengineering, mangrove autoecology, and sea-level rise interacted across the late-20th century to accelerate the creation of novel seascapes.

Introduction

Across the neotropics, coastal wetlands are transforming in response to climate change and sea-level rise (Harley et al., 2006). At various global locales, mangroves are encroaching into salt marshes and transforming the ecology of tidal ecosystems (Saintilan et al., 2014). While oscillations between woody and herbaceous vegetation boundaries are common and well-documented in many ecosystems (Knapp et al., 2008, Van Auken, 2000), widespread marsh-to-mangrove conversion has accelerated in recent decades. Such conversion has caused alarm among ecologists and coastal resource managers (e.g., Armitage, 2015; Everitt et al. 2010; Perry and Mendelssohn, 2009; Williamson et al., 2011), who recognize that major shifts in foundation vegetation taxa will likely drive commensurate qualitative and quantitative changes in the ecosystem services provided by coastal habitats (see Engle, 2011). Further, salt marsh and salt prairie environments are widely recognized as critical habitats for terrestrial mammals, reptiles, amphibians, mollusks, migratory birds, and waterfowl not typically found in mangrove forest (Montague and Wiegert, 1990:508−512; Odum and McIvor, 1990:542−543).

Workers have identified various drivers of mangrove expansion and encroachment, including sea-level rise (Krauss et al., 2011, Lopez-Medellin et al., 2011, Rogers et al., 2006, Smith et al., 2013), warming winter temperatures (Duke et al., 1998, Osland et al., 2013), elevated atmospheric CO₂ (McKee et al., 2012), precipitation (Saintilan and Wilton, 2001), droughts (Rogers et al., 2006), and sedimentation (Woodroffe et al., 1985). At local- to meso-scales, however, patterns of mangrove expansion into adjacent habitats may be influenced substantively by interactions between climate change and direct anthropogenic impacts (He and Silliman, 2019, Patterson et al., 1997, Patterson and Mendelssohn, 1991). Such interactions will likely produce novel types of tidal wetlands (sensu Hobbs et al., 2013; Lugo et al., 2014), characterized by new species combinations and biophysical interactions that lack local historical analogs.

In 2012, Raabe et al. (2012) compared 19th century survey documents with contemporary aerial photography to infer a previously undocumented estuary-wide pattern of marsh to mangrove conversion within the tidal zone of Tampa Bay Estuary on the central peninsular Gulf Coast of Florida. The authors reviewed historical Coast and Geodetic Survey (CGS) Topographical Charts (T-Sheets) and General Land Office (GLO) survey records to generate models of late-19th century coastal vegetation cover within four areas of Tampa Bay Estuary. They compared these historical models to modern vegetation cover assessments based on 1999 aerial imagery (SWFWMD, 2005) and documented a dramatic trend of mangrove encroachment in each of the study areas (Raabe et al., 2012:1153). While latitudinal temperature gradients are known to condition the physiographic distributions of mangrove and salt marsh along the Florida peninsula and the Gulf of Mexico (Kangas and Lugo, 1990, Osland et al., 2013, Stevens et al., 2006), the latitudinal pattern of wetland conversion documented by Raabe et al. (2012:1155) within Tampa Bay Estuary does not implicate temperature change as a major factor. Instead, their interpretation broadly attributes mangrove encroachment to sea-level rise and suggests that impacts of coastal urbanization (e.g., drawdowns of local aquifers, damming of streams, and shoreline alteration) also functioned as important drivers (Raabe et al., 2012:1156).

While the comparative historical analyses by Raabe et al. (2012) is both rigorous and compelling, to date there are no published sediment records that ground-truth tidal wetland conversion in Tampa Bay Estuary. Further, while 19th century survey documents provide essential points of historical reference, they are not particularly suitable for investigating the chronology or process of mangrove encroachment across the 20th century. In their discussion, Raabe et al. (2012:1156) called for further comparative analyses of historical aerial photography to understand better the nature and timing of habitat transformation. As a complement to the research reviewed above, the present study integrates sedimentological and remote-sensing approaches to clarify 20th century marsh-to-mangrove conversion. We focus on two study sites located within Old Tampa Bay—the western basin of Tampa Bay Estuary. Building on Raabe et al. (2012), we provide new data from sediment core samples that ground-truth wetland conversion patterns observed in historical map comparisons. These new data enable refinement of the chronology of habitat transformation. To contextualize the sediment records and investigate the processes driving marsh-to-mangrove conversion we analyzed sequences of 20th century aerial photography at each study site and correlated sediment core records with changes apparent on the larger seascapes.

This article thus addresses the following research questions. First, what near-historical (20th century) changes in habitat type do the tidal wetland sediments of Old Tampa Bay reveal, and how consistent are these records with current understanding of marsh-to-mangrove conversion? Second, what is the precise chronology of these transformations as recorded in the sedimentary record? In our discussion of these results, we evaluate the major factors that drove the process of wetland conversion at the study sites in Old Tampa Bay (sections 5.1 and 5.2). Additionally, we discuss the implications of marsh-to-mangrove conversion processes for the conservation and management of coastal wetlands (section 5.3).