INDICATORS OF SALTWATER INTRUSION IN DOG RIVER

 

Amy Milam, Department of Earth Sciences, University of South Alabama, Mobile, AL 36688.  E-mail: a.milam@mchsi.com.  For the coastal community of Mobile, Alabama, and for the ecology of Dog River, saltwater intrusion is a significant concern; and, currently, there is no established monitoring system or continuous historical data for the river.  In addition, research on the ecology and dynamics of tidal freshwater streams and estuary systems such as Dog River has largely been neglected as they straddle the divide between limnology, freshwater ecology, and marine ecology.  Yet they are of great importance to coastal ecosystems and communities.  Saltwater intrusion creates losses of tidal marshes and associated wildlife, contamination of freshwater resources, and degradation of the immediate shoreline.  This study utilized Bald cypress (Taxodium distichum L.), as an indicator for saltwater intrusion of Dog River because it is the longest lived, dominant tidal marsh species in the southeastern United States and is common in the wetlands of Mobile, AL.  Bald cypress can tolerate some fluctuations in salinity but not a consistent inundation of water with salinity over 2-3ppt salinity.  In addition to observing bald cypress, transitional marsh vegetation was documented and salinity measurements were taken at sites throughout Dog River.  Though saltwater intrusion was not found to be severe in Dog River, the results indicate that slight increases in the salinity of Dog River have occurred over time.

            Keywords: saltwater intrusion, Bald cypress (Taxodium distichum L.), Alabama

 

 

Introduction

Saltwater intrusion is the introduction, accumulation, or formation of saline water into a water of lesser salinity.  The processes involved can be natural or human-induced but are particularly exacerbated by human activity such as urbanization, dredging and other various modifications in the hydrology of tidal streams.  The result of this is the ultimate loss of tidal marshes and associated wildlife, contamination of freshwater resources, and degradation of the immediate shoreline (State of LA 1993). In places such as Louisiana, Florida, Australia and China where saltwater intrusion is already occurring, economic losses are being incurred due to declines in numbers of commercially important fish and shellfish, the loss of suitable agricultural land, diminished property values, the cost of preventative methods such as barriers to saltwater, and the cost of treatment for contaminated groundwater (State of LA 1993, White 1996, Ross, et al. 1994, Pierce, et al. 1993).  For the coastal community of Mobile, Alabama, and for the ecology of Dog River, saltwater intrusion is a significant concern, as it is for the entire northern Gulf coast, which produces 25% of the United States’ commercially harvested seafood (O’Bannon 2001).  Dog River is a part of one of the richest coastal areas in the United States in terms of biodiversity and the seafood industry (UOCS 2005).  It is of exceptional importance not only to the water quality and ecological function of the bay and coastal ecosystems, but to the Mobile Bay community.  Dog River alone drains almost 100 square miles of Mobile County and is a tributary of the Mobile Bay estuary system.  Dog River has actually been considered by some in the scientific community to function more as an estuary than an actual river (Eddins 2001).  Estuaries are semi-enclosed coastal bodies of water, which have free connections with the open sea and within which seawater mixes with fresh water.  Estuaries are among the most productive ecosystems on Earth and serve as nurseries for fish and shellfish, most of which are commercially harvested or coveted by fishing enthusiasts.  One of the reasons the Gulf is so productive is because of its many estuaries. 

Pressures on Dog River and other tidally influenced freshwater streams and estuaries largely come from urbanization, improper land use practices, impervious cover, dredging and any change to the hydrology of the stream.  Climate change, which can lead to sea level rise, presents another stressor that is often exacerbated by at least one of the aforementioned activities.  One outcome of all of these pressures, saltwater intrusion, is unique to these tidally influenced systems (Vile et al. 2005).  Tidal freshwater environments rarely reach salinity levels over 0.5ppt and a sustained increase in salinity poses a threat to the viability of vegetation and wildlife (Pasternack 2006).  When saltwater intrusion occurs, it degrades overall ecological function and contaminates freshwater resources.  More specifically, increased salinity disrupts biogeochemical interactions such as the transport and sequestering of carbon [C] and phosphorous [P] (Vile et al. 2005).  Salt sensitive vegetation begins to diminish, resulting in rapid erosion of marsh shoreline soils and eventually the conversion of marsh to open water.  This degradation has been documented in Louisiana where the loss of at least 32,000 acres of wetlands is occurring each year (State of LA 1993). 

The vegetation indicative of tidal freshwater streams and wetlands varies from salt-tolerant or halophytic species to halophobic (freshwater dependent) vegetation.  Observations of changes in vegetation along tidal freshwater streams where no historical water quality data exist may indicate evidence of saltwater intrusion.  A case study in the Florida Keys, observed a decline in Slash pine (Pinus elliottii var densa) forests thought to be due to saltwater intrusion as a result of rising sea-level.  The results showed that the pine forests had been and were being replaced by more halophytic plant assemblages such as mangroves due to salinization (Ross et al. 1994).

Where Dog River is concerned, saltwater intrusion could be occurring due to the application of impermeable surfaces in the Dog River Watershed. Because of concrete drainage ditches and asphalt parking lots and roads, it is likely that saltwater is moving farther upstream.  In addition, Dog River’s major freshwater source, Wragg Swamp, has been drained and dried up for over 50 years.  Based on these factors, it is reasonable to expect an increase in salinity over time and gradual, subtle increases can still have the same detrimental repercussions for Dog River.  Local fishing enthusiasts have recently noted catches of predominantly salt to brackish water fish, such as speckled trout, farther up stream than usual.  However, in view of the fact that historical salinity measurements for Dog River are rare, this type of evidence is circumstantial without scientific research.  What does exist are salinity measurements taken as far back as 1998 by Alabama Water Watch Volunteers at several sites throughout Dog River and its tributaries.  This data does help to set a baseline but it is still subject to the volunteer and most of the data for each site is discontinuous; no sites are permanent.  Thus, at this time it is insufficient data to determine any change in salinity and or saltwater intrusion.  A possible solution to this problem is to assess the riparian (shoreline) vegetation for clues, which is the heart of this study.

Specifically, I have used Bald cypress (Taxodium distichum) as an indicator of salinization in Dog River.  Bald cypress has a low salt tolerance of 2-3ppt at best and occurs in all freshwater tidal streams as well as many non-tidal freshwater streams throughout the southeast (Penfound & Hathaway 1938, Brown & Montz 1986).  Flood inundation for prolonged periods can lead to the death of bald cypress trees, however, along the Dog River edge this should not be the cause of death because the dynamics of Dog River and its relatively short length from headwaters to mouth allow little room for prolonged flooding.  Historical flooding of the Dog River has lasted no more than a couple of days after a severe storm and is insufficient time to cause death of entire stands of bald cypress trees.

 

Research Question

Is saltwater intrusion occurring in the Dog River tributary of Mobile Bay? 

The goals of this research are to add to current salinity measurements for Dog River to help implement a baseline for salinity in Dog River, and to assess the utilization of Bald cypress trees, as indicators for salinization in tidal freshwater streams.

 

Methods

The initial idea for this project was in areas along the river where water sampling yields consistent levels of salinity above 2-3ppt (the maximum for bald cypress) and bald cypress are observed dead or existing with halophytic vegetation may indicate that the area was once predominantly freshwater and has been overcome by saltwater intrusion.  A bald cypress tree was presumed dead if there was no new growth in the form of leaves and no evidence of leaves in previous years (Figure 1).  An initial survey of Dog River and its three main tributaries, Halls Mill Creek, Eslava Creek, and Rabbit Creek was conducted from a small boat in March to make observations of bald cypress and dominant marsh vegetation.  Black needle rush (Juncus romerianus) and Smooth cordgrass (spartina alterniflora) are halophytic marsh species that were observed as markers for saline conditions versus cattails (Typha latifolia), wild rice, and many herbaceous plants as indicators of predominantly freshwater areas (Odum, 1988).  Surface water samples were taken for salinity measurements and GPS waypoints made at 13 sampling sites along the river (Figure 2).  The sites were as follows:

At each site, water samples were taken at the bottom, middle, and top of the water column.  A small plastic vial  was filled with water from the sample then the vial was marked with a number.  Each site was represented by three vials.  For example, from Site 1, the first vial was labeled with the number one for the bottom sample of Site 1, the next vial was labeled number 2 for middle sample of Site 1, and the next vial was labeled number 3 for the surface sample at Site 1.  The vials for Site 2 then would be represented by numbers 4, 5, and 6 and so on until 39 samples were taken for 13 sites.  The 39 water samples were taken back to the laboratory and allowed to come to the same temperature for at least 12 hours.  A refractometer was used to measure salinity for each sample, then the measurement in parts per thousand (ppt) was recorded in a notebook and later put into and Excel worksheet. A waterproof note pad and pencils were used to write observations, salinity measurements, and other notes while on the river.  A digital, 5 mega-pixel, Olympus camera was used to take pictures of significant observations.

In selected cypress stands a ground water sample was taken by digging a hole to the water table, letting the water percolate into the hole and filling a small vial with the water to measure for salinity later.  A GPS waypoint was made to designate the site and the vial was marked with the name of the waypoint.  These sites were:

A GPS waypoint was made for a site that was not sampled but was observed on the last sampling trip as having possible salt intrusion due to the observation of significant black needle rush with little to no ground dwelling freshwater plants observed but with bald cypress present.  Some of the cypress were dead and many were unhealthy in appearance by not exhibiting much new growth.  There was no evidence of regeneration of cypress at this site.  This site was:

Four sampling trips were made to Dog River by boat at varying times of high, low, and neap tides.  Wind conditions were later recorded because winds will have an effect on salinity as well as tides.  For example a strong North wind will fight against a high tide, pushing water out of the river, reducing salinity levels.  The first trip was made on March 25, on the rising tide; samples were taken from 9:30AM to 5:15PM with high tide at 1:05P.  Wind direction and speed on March 25, was North-Northwest at 24 knots.  The second trip was made on April 2, starting just before high tide to take the majority of samples at high tide, which occurred at 1:53PM.  Wind direction and speed on April 2, was Southwest at 23 knots.  The third trip was made on April 8, to get samples at low tide, which occurred at 6:24AM.  Wind direction and speed on April 8, was South-Southwest at 33 knots.  The fourth and final trip was made on April 9, at a neap tide; samples were taken from 12:30 to 4PM.  Wind direction and speed on April 9, was North at 25 knots.  The Alabama Department of Conservation and Natural Resources, the United States Geological Survey in Montgomery, Alabama and the Alabama Water Watch Headquarters in Auburn, AL were contacted to provide historical water quality data.  The only organization that could or would provide this data was AWW.  A search for historical images (photographs, aerial photographs, etc.) of Dog River was also conducted to assess the previous extent and condition of vegetation on the river.  Many historical photos of residences and areas along Dog River were found but none that showed clearly the types of vegetation along the shoreline.  I did take pictures while I was sampling of areas representative of freshwater vegetation, salt to brackish water vegetation and areas that showed significant decline of bald cypress with significant increase in saltwater vegetation.  Finally, I compared my salinity measurements with those provided by AWW and compared observations of vegetation occurring along the river with the salinity measurements.

 

Results

On March 25, salinity measurements ranged from 2-7ppt. Salinity measurements for each of the four sampling days can be observed in Figure 3.  At high tide on April 2, the measurements ranged from 1-5ppt.  At low tide on April 8, salinity ranged from 2-5ppt.At a neap tide on April 9, salinity measurements ranged from 2-5ppt. The observations made at each site combined with salinity measurements were as follows:

 

Site 1-predominantly salt-tolerant freshwater vegetation.  Black needle rush was the dominant plant form. The median salinity measurement here was 5ppt with as low as 3ppt and as high as 7ppt.
Site 2-predominantly freshwater vegetation with very few bald cypress evident in a patchy distribution.  Other trees such as pines, oaks, wax myrtle, gum, sweet bay and other typical wetland trees and shrubs were in dense assemblages between private lots.  Also, the lots that did exist were bulkheaded in one way or another and had little to no marsh vegetation.  The median salinity measurement here was 4ppt with measurements as low as 3ppt and as high as 6ppt.
Site 3-The confluence of includes freshwater draining from Rabbit Creek and Halls Mill Creek, Moore Creek, and Robinson Bayou while receiving saltwater influx from the Mobile Bay. Observing a representative or dominant vegetative cover was difficult due to the nature of the site and its geographic location. The median salinity measurement here was 4ppt with measurements as low as 3ppt and as high as 6ppt. A couple of dead cypress were observed at the confluence and a ground water sample taken where they stood which yielded a salinity measurement of 8ppt.  This site was labeled Site C.  The high salinity measurement here reflects higher salinity measurements of surface water in the recent past.  It is such a higher measurement than the water because salt takes longer to dilute out of soils than in water thus the ground water measurements do not fluctuate with the tide on a daily basis.
Site 4- predominantly cypress stands along both sides of the creek with black needle rush lining the shore underneath them.  Mixed in with the black needles rush were some freshwater herbaceous plants such as arrow arum, spider lily and blue flag.  There were some dead cypress with in the stands but most were alive and some regeneration was evident.  Here it will be noted that at the mouth of Rabbit Creek predominantly salt-tolerant vegetation black needle rush and smooth cord grass with a few individual dead cypress was observed (Figure 4).  The median salinity measurement here was 4ppt with measurements as high as 5ppt and as low as 2ppt.
Site 5-predominantly freshwater vegetation; trees, shrubs and some freshwater herbaceous plants on the ground.  Chinese Tallow trees were flourishing with bay, pine, tupelo-gum, black willow, myrtle and others. The median salinity measurement here was 2ppt with measurements as high as 3 and as low as 1. However, there were barnacles (saltwater crustaceans) on a metal ladder attached to a dock.

Site 6-predominantly salt-tolerant vegetation observed here; black needle rush with a few dead cypress on the edges of the shoreline (Figure 5).  There were two anglers here who said that they have been fishing in Dog River for 17 years and have been catching more saltwater fish such as speckled trout. They also said that the bass seem to be doing fine because they are still catching up to 6-pound bass in Perch Creek, Rabbit Creek and other areas of the river. The median salinity measurement here was 3ppt with no variation in all 12 samples.

Site 7-predominantly freshwater vegetation observed, much like Site 5 on Rabbit Creek with trees and shrubs and freshwater herbaceous species. An osprey was nesting in an old dead cypress tree at this site. The tree appeared to have died from rising water levels as it was on the extreme edge of the shoreline and its roots were completely under water. It appeared as if it was at one time on land as it was one of the largest cypress trees in the area. The median salinity measurement here was 2ppt with the occasional measurement of 3ppt.

Site 8-all shorelines in the immediate area were bulkheaded and the dominant riparian vegetation present was that of pines and oaks. This is a heavily developed part of the river adjacent to the River Yacht Basin, also known as the old hunting and fishing club.  The median salinity measurement here was 4ppt with measurements as high as 7ppt and as low as 3ppt.

Site 9-predominantly salt-tolerant vegetation, black needle rush and smooth cordgrass, along the immediate shoreline while cattails were observed behind the saltwater vegetation separated by a canal 20-30 feet in width.  Some cypress were observed to the south with no dead cypress and to the north side of the creek was a dense assemblage of shrubs and other trees. The median salinity measurement here was 4ppt with measurements as high as 5ppt and as low as 3ppt.

Site 10-this was the site of what appeared to be the vegetative transition zone along the body of Dog River heading towards Navco/Luscher Park. There was a mix of smooth cordgrass, black needle rush, cattails, wild rice. Further up cypress became more prolific and the stand grew denser with advancement upstream. The part of the river between this site and Navco, however, displayed many felled cypress.  The median salinity measurement here was 3ppt with a couple of measurements of 4ppt.

Site 11- predominantly freshwater vegetation, including cattails, wild rice, and several herbaceous plants.  Live cypress were also present. The median salinity measurement here was 3ppt with a few measurements of 4ppt as well.

Site 12- some live cypress were present but this is an area where there is heavy bulkheading and not much riparian vegetation. The observation was made that no salt-tolerant vegetation was thriving here, instead there was more wild rice type marsh grasses. The median salinity measurement here was 2ppt with an almost equal number of measurements at 3ppt.

Site 13-predominantly freshwater vegetation thrives at this site with no occurrences of salt-tolerant vegetation. Mostly grasses and some herbaceous ground cover dominated the riparian zone with a Chinese tallow trees flourishing and almost no cypress sightings.  The median salinity measurement here was 2ppt with no variation in all 12 samples.

            It should be noted that Mobile County has been under a dry spell and the only day that received rain during the course of the project was April 8 from 6:00AM to 7:30AM. On this day, the city of Mobile experienced heavy rain and Dog River received light rain.


Conclusion

For the coastal community of Mobile, AL and for the ecology of Dog River, saltwater intrusion is a significant concern.  Intrusion of saltwater along Dog River would create economic problems for the coastal community in a variety of ways depending on the severity of the intrusion from decreasing property value to loss of habitat for commercial and recreational species of fish and shellfish.  The Dog River community would also suffer from loss of natural vegetative protection from storms, contaminated ground water resources and a synergistic enhancement of the toxicity of dissolved chemicals in the river.  (State of LA 1993).  All those concerned should realize that Dog River’s major source of water now is from Mobile Bay and that any change in the limnology of the bay will be paralleled in Dog River on a smaller scale.  The weather buoy in Mobile Bay has recorded salinity as high as 20ppt. 

Based on the data presented in this study combined with the knowledge of the salt tolerance of bald cypress (2-3ppt), it would be reasonable to note a slight increase in salinity of Dog River in some areas where freshwater vegetation had previously dominated the shoreline.  This is evident by the fact that there are numerous dead and struggling bald cypress standing along the shoreline of Dog River among black needle rush and smooth cordgrass, plants that tolerate much higher levels of salinity.  When water samples were taken in these locations they consistently yielded salinity of 3ppt or higher, pushing the limits of bald cypress salt tolerance.  More tests on leaf ion content and photosynthetic ability of bald cypress could be taken to make an absolute verdict as to the actual cause of death for the dead cypress.  The results do not show a significant salinity increase in the river as a whole unit and salinity measurements are well within the limits for a tidal freshwater system such as Dog River (<5ppt). 

The results of this study will help Dog River residents, the scientific coastal community, the Alabama Coastal Foundation and other organizations to identify a need for consistent monitoring of salinity on Dog River and to begin looking at what can be done to combat any threat of saltwater intrusion in the future.  Some suggestions to this pending concern are to reassess impermeable drainage surfaces in the Dog River watershed and explore options for replacing them with more permeable surfaces such as riprap.  In addition, opportunities for preserving any sources of freshwater flow into Dog River should be assessed and seriously considered including locating fresh ground water flows into Dog River, as they may be the last real source of freshwater left for the river other than rainwater.  A consistent monitoring system for the river, such as a weather bouy should be considered.  The Alabama Department of Environmental Management or the Alabama Department of Conservation could possibly add a site in Dog River, perhaps at the confluence, to their routine sampling efforts. 

Where no historical salinity data exists, scientific research of vegetative indicators for the salinization of tidal freshwater streams can yield clues to the historical extent of freshwater downstream and the upstream movement of the saltwater-freshwater interface in tidal freshwater systems.  This study can be used in combination with Alabama Water Watch salinity data and any other salinity data for Dog River that may be made available in the future, to provide a baseline of salinity measurements for Dog River.  This study documents an effective way for future studies to be carried out on other tidal freshwater streams along the Gulf Coast and in other parts of the world.  Dog River Clearwater Revival and other coastal organizations such as the Alabama Coastal Foundation can use this study for educational or informative purposes and to enhance decision-making.  For example, the Alabama Coastal Foundation (ACF) holds an annual bald cypress tree planting as part of its marsh restoration project.  ACF would be able to make better-educated decisions on the best and worst sites to plant bald cypress based on salinity.

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