Brackish marsh vegetation distribution is strongly controlled by inundation and salinity, and is likely to be impacted by climate change and sea level rise due to changes in the periodicity and magnitude of freshwater and salt water inputs. With increases in sea level, shifts in dominant marsh species would be expected, altering typical brackish marsh plant diversity and moving these systems towards saltier marsh vegetation. Yet little is known about the effects of these hydraulic regimes on brackish marsh floral communities in northern New England. Quantifying vegetation response in these systems to inundation and salinity will lead to stronger predictions of change when considering long-term protection and conservation of these fragile habitats. The goal of this study is to construct a model as a guide for predicting vegetation response to inundation and salinity along a brackish gradient using experimentally obtained information on plant responses to these different regimes.
Estuaries are important biological habitats in the Gulf of Maine, providing nursery grounds for commercially important species, and habitat for many or endangered species. Estuarine marshes provide a natural filter that keeps excessive nutrients and discharge from entering our rivers, lakes, and oceans (Maine DEP 1996; NECIA 2007). Between the ocean and our bodies of freshwater lay marshes that experience salinities at both ends of the spectrum, seasonally or tidally (Odum 1988; Touchette 2006). These brackish marshes experience significant mixing of freshwater and seawater inputs, and salinities can sometimes reach fresh conditions (Tiner 1987). These marshes are likely to be highly impacted by climate change and sea level rise due to changes in the periodicity and magnitude of freshwater and seawater inputs. They share many characteristics of freshwater wetlands, like plant species (Meyerson et al. 2000; Latham et al. 1994; Gallagher and Kibby 1980). For our study, we will focus on these brackish marshes, which range from mesohaline marshes (5-18 PSU) to oligohaline marshes (0.5-5 PSU) (Odum 1988). Brackish marsh vegetation patterns are strongly controlled by their hydraulic regime (Meyerson et al. 2000). Within the estuary, salinity is typically considered the determining factor for most plant species’ distribution (Odum 1988; Meyerson et al. 2000). However, sediment anoxia (brought on by periods of inundation) is a greater stress on brackish and freshwater marsh plant species (Stribling et al. 2006). Understanding these succession dynamics across salinity and tidal inundation gradients in marsh systems is critical for restoration/ management efforts (Weiher and Keddy 1995; Crain et al. 2004; Konisky and Burdick 2004).This fact is particularly true in context of predicted rises in sea level (Solomon et al. 2007; NECIA 2007), because gradients in salinity and inundation change species’ composition and environmental characteristics of estuarine marshes (Weiher and Keddy 1995; Crain 2008; Crain et al. 2008; Cooper 1982). Plant-water relationships under current changing conditions are crucial to expand what we know about salinity resistance in plants (Touchette 2006).
Structuring of species interactions and marsh plant community composition and abundance by environmental gradients is well known for salt marshes (Bertness 1991; Weiher and Keddy 1995; Crain et al. 2004; Crain et al. 2008). However, these dynamics have not been well characterized for brackish marshes, as few experiments have tested increased salinity and inundation on growth limits, and are limited in their scope (Bertness and Ellison 1987; Burdick et al. 1989; Hellings and Gallagher 1992; Konisky and Burdick 2004; Meert and Hester 2009). Predictive models are important for understanding species interactions in the context of community structure and physical environments across natural abiotic gradients (Paine and Levin 1981; Sousa 1984; Brooker 2006; Gedan et al. 2009). To address the increasing difficulty of understanding what changes (retreat of habitat, species composition, etc) to expect with rising sea levels or a resource management change in tidal hydrology, a broader view is necessary to understand relationships between hydrological and elevation characteristics and vegetation change. Konisky and Burdick 2004 address this question for marshes along a gradient from mesohaline to polyhaline marshes. In the face of predicted sea level rise (or changes in hydrology due to management actions), the question is: how will changes in hydrology (i.e., tidal inundation and soil salinity), impact vegetation distribution in Maine brackish marshes? In this project, I propose to determine interactions of hydrology and vegetation in brackish systems across a gradient of soil salinities from mesohaline to oligohaline and create a model that details interactions for restoration and resource management purposes. I hypothesize that increases in salinity and increases in tidal inundation should have a negative impact on plant growth, but I expect that increased tidal inundation will have the greatest effect on high marsh plant species of either salinity regime.
Experimental sites will be chosen in May based on average soil salinities (Konisky and Burdick 2004). There will be two sites, stratified by expected salinity, with four 1m2 plots placed in low and high marsh areas. Inundation and salinity effects will be measured. The sites will be placed in existing bare patches, or black-plastic sheets will be placed to inhibit growth two weeks prior to the start of the experiment.
Intact plants of two common mesohaline species (Spartina alterniflora- low marsh; Spartina patens- high marsh) and two common oligohaline species (Agrostis stolonifera- low marsh; Scirpus robustus- high marsh) will be transplanted into open bottom pots with 2 individuals/pot and placed in groups of 18 pots/plot, with pore water salinity wells placed between each plot at 5 to 20 cm belowground to measure localized soil salinity for each site (Neckles et al. 2002). Species will be randomly paired in each pot to assess competitive interactions under different salinity and inundation regimes. Three individuals of each species will be measured for initial height, then collected, dried, and weighed to estimate initial above/belowground biomass. Every other week from May–September, pots will be monitored and weeded. I will also measure pore water salinity, number of additional shoots, shoot height, and shoot survival. In September, surviving plants will be extracted, dried and weighed for final above/belowground biomass.
Inundation depths, duration, and temperature will be quantified with in-situ dataloggers at each site over 1-3 neap/spring cycle(s) (Neckles et al. 2002). Loggers will be suspended at 1 m depth. Elevation of each logger station (and plot) will be collected with rod-and-level survey techniques and will be referenced to the North American Vertical Datum of 1988 (NAVD88). An iButton temperature datalogger will be placed in each plot, which are an effective indirect measure of tidal inundation (Gjerdrum et al. 2008). These measurements will be used to estimate a tidal curve, determining duration/depth of tidal inundation of each site. Soil redox potential will be measured to account for the controlled effect of competitive interaction, as well as length of time of anoxic conditions (periods of inundation) (Stribling et al. 2007).
Impact of Project
Little information is available to predict the response of brackish marsh ecosystems to climate change, because these systems are understudied (Stribling et al. 2006). Quantifying vegetation response to inundation in these systems will lead to stronger predictions of change when considering protection/conservation of these diverse habitats, and expand the current knowledge of salinity tolerance of plants in northern New England estuaries. This project provide critical reference data for on-going restoration efforts for nearby Sherman Marsh.
Elizabeth Mitchell, M.S. Student in Biology
University of Southern Maine
Portland, ME 04103
Dr. Karen Wilson (faculty advisor)
University of Southern Maine
Portland, ME 04103