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GCPO LCC staff recently finalized the second draft of the Ecological Assessment of Estuarine Tidal Marsh as one of the nine priority systems highlighted in the GCPO LCC Ecological Assessment project.  This draft incorporated reviews by coastal marsh experts in the GCPO geography as well as LCC staff in addition to data revisions suggested in the GCPO Conservation Blueprint workshop in Spanish Fort, AL this past spring.    

The GCPO LCC is working to design a regional network of lands and waters capable of sustaining natural and cultural resources now and into the future.   This conservation blueprint effort is built upon the premise that we understand the current state of each priority ecological system identified by the GCPO Adaptation Science Management Team in the 2014 draft Integrated Science Agenda (ISA).  The ISA identifies desired ecological states for priority systems through a series of endpoints related to landscape features and species indicative of a healthy ecological system.   The Ecological Assessment project uses existing digital geospatial data to assess three basic themes as they relate to desired ecological states of each system: How much of the ecological system is in the desired ecological state?  How much more is needed?  Where is the system already in the desired ecological state and where are opportunities to manage for these conditions? 

Estuarine Tidal Marsh

Estuarine tidal marsh is one of two Initial ISA-defined priority systems along the GCPO Gulf Coast (the other being beaches and dunes).  The desired ecological state for estuarine tidal marsh is defined as “stable marsh systems comprised of native vegetation and limited open water conditions occurring in large blocks with natural hydrology present”.  Landscape endpoints indicative of this desired state relate to measures of emergent and submergent vegetative cover, coverage of open water, presence of native vegetation and barrier islands, connectivity and contiguity of marsh in large blocks with moderate edge, and reflecting natural gradients of salinity, freshwater flow, and tidal influence. 

Through the assessment we estimate there are 202,584 acres of estuarine tidal marsh within the GCPO portions of the Gulf Coast, which includes the western Florida Panhandle, Alabama and Mississippi, and portions of southeastern Louisiana (Fig. 1). Of that amount we estimate 51% (103,538 acres) is currently protected.  We estimate a net loss of 15,116 acres of estuarine tidal marsh in the GCPO from 1996-2010, with the majority attributed to loss to open water, primarily in Louisiana, though over a thousand acres are estimated to be lost to development (Fig. 2). 

We found 144 large patches (>250 acres) comprised 73% of all patch acreage in the GCPO, suggesting a relatively small number of large patches hold a disproportionate amount of tidal marsh acreage. Nearly all (98%) of large patches are considered “unbroken”, exhibiting >70% emergent vegetative cover and <20% open water.  We found marsh types within patches to be sufficiently interspersed, with an average 27% saline, 40% brackish, and 31% intermediate marsh within patches in Alabama, Mississippi, and Louisiana.  Composition varied by geography, with marshes in Louisiana exhibiting different composition of marsh types compared to those in Mississippi, and Alabama (Florida marsh type data were not available).  We also found the majority of large patches exhibited “moderate” amounts of edge, with marshes in southeastern Louisiana demonstrating greater edge densities compared to other states.  We also estimate 34,501 acres of submerged aquatic vegetation (SAV) with 84% of the total found in GCPO portions of Florida, and very little found nearby estuarine tidal marsh patches. Additionally we summarized existing data related endpoints targeting coastal barrier islands, native marsh vegetation, freshwater inflow, and salinity levels, though this data was not included in the development of condition indexes described below.  

Estuarine Tidal Marsh Condition Index

The goals of the ecological assessment of estuarine tidal marsh were to determine where in the GCPO Gulf Coast estuarine marsh systems exist in or nearly-in the desired ecological state outlined in the GCPO LCC Integrated Science Agenda.   To accomplish this we used quantifiable landscape endpoint data on patch size, interdigitation of marsh types, edge, emergent vegetative cover, open water, and submergent vegetative cover to calculate a series of condition index values (CIV) as a baseline for assessing amount of marsh within or near the desired ecological state for this system.  We used an additive score-based decision tree approach to calculate CIV scores from 0 to 17 for each marsh pixel based on the number of landscape endpoints met.  Marsh pixels found in large (>250 ac) patches and considered unbroken scored a CIV from 14-17 depending on how many additional endpoints were met. An index value of 17 represents marsh pixels that are estimated to be in the desired ecological state, as determined by the suite of measurable condition endpoints.

We found 67% of tidal marsh acres in the GCPO geography were in large (>250 ac) unbroken patches and met one or two additional landscape endpoints targeting desired ranges of edge, interdigitation, or submergent vegetative cover (CIV =15 – 16; Fig. 3).  However, we found no areas on the landscape that met all measurable landscape endpoints (CIV = 17), primarily due to limitations of endpoint data.  We estimate 32% of marsh acreage to be found in unbroken patches that are <250 ac in size (CIV=6-9), suggesting significant potential to target marsh conservation and restoration efforts to strategically connect small unbroken patches to each other or to existing large patches to strengthen connectivity of marsh systems along the coast. 

Understanding the current state of marsh systems provides a critical input layer into the GCPO LCC Landscape Conservation Blueprint in combination with information on existing conservation investments, partner priorities, potential threats, and species-habitat associations to create a blueprint for large-scale conservation efforts into the future.  This information will provide a vital foundation to aid Gulf Coast conservation practitioners in making informed decisions regarding where and how much conservation action is needed to support healthy Gulf tidal marsh systems.  This assessment is also intended to identify regional information gaps and data needs, whereby the GCPO LCC can work with the partnership to set the course for addressing information needs.

For more information or to complete a review download the Tidal Marsh Assessment in Brief, full Ecological Assessment Report for Estuarine Tidal Marsh, or explore the Marsh Assessment Data Gallery on the GCPO Conservation Planning Atlas.

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