FINAL REPORT Sargassum - Spatial Sciences Laboratory

FINAL REPORT Sargassum: Erosion and Biodiversity on the Beach Rusty A. Feagin1 & Amy M. Williams1 1
Spatial Sciences Laboratory, Department of Ecosystem Science & Management Texas A&M University, 1500 Research Pkwy., Ste. B223, College Station, TX 77845 A report of the Coastal Coordination Council pursuant to National Oceanic and Atmospheric Administration Award No. NA06NOS4190219. This work was supported by National Oceanic and Atmospheric Administration (NOAA) and Texas General Land Office (GLO) grant #07‐005‐10. 1 This work was supported by National Oceanic and Atmospheric Administration (NOAA) award # NA06NOS4190219 and Texas General Land Office (GLO) grant #07‐005‐07. Contact Information: Dr. Rusty Feagin Spatial Sciences Laboratory Dept. Ecosystem Science & Management Texas A&M University 1500 Research Parkway, Ste. B223 College Station, TX 77845 +01‐(979)‐862‐2612 (phone) +01‐(979)‐862‐2607 (fax) [email protected] (email) 2 Summary This study looks at the geomorphic and ecological impact of cleaning Sargassum spp. off of the forebeach on Galveston Island, Texas. The results show that beach raking did not significantly change the elevation of raked beaches, when compared with unraked beaches, over a two year time period. The study also indicates that although there are many pelagic species found in the sargassum, most pelagic fauna are dead and not using the sargassum for habitat once it is on the beach. Raking and depositing the sargassum into new locations likely affects the terrestrial Orchestia spp. the most negatively (a detritivore ‘fly’). In other studies, we have found that raking sargassum and then depositing it at the base of the dunes helps dune vegetation grow; the mechanism of this enhanced growth is through the input of these nutrients, primarily nitrogen and phosphorus. Thus, beach raking may be an adequate method for managing sargassum; its ecological benefits are most strongly realized when it first washes up and again, when subsequently placed upon embryonic dunes by humans. However, detritivores are likely impacted by this management activity. More research is needed into the effect of increased tractor traffic upon upon crabs, and into the economic impacts of sargassum upon recreational preferences and beach tourism. 3 Introduction Sargassum fluitans and S. natans, two species of brown algae commonly known as gulfweed or sargassum, grow in dense intertwined mats in calm tropical and subtropical waters (TANAKA and FOSCA, 2003; GOWER et al., 2006). Sargassum deposits near the high tide line and piles up along the beach (GHESKIERE et al., 2006) on Galveston Island, Texas each year from May to August (Figure 1). Large piles of sargassum restrict human access to the beach and water. Sargassum traps material such as seeds, animals, decaying matter and anthropogenic litter from the water column, which are also deposited on the beach (COLOMBINI and CHELAZZI, 2003). Human generated trash can be hazardous to beachgoers (i.e. medical wastes, construction materials). Furthermore trash entangled in the sargassum wrack is considered unsightly. Since the deposition of sargassum occurs in the middle of Galveston Island’s peak tourism season, coastal managers are confronted with the difficult choice of cleaning sargassum off the beach (economic reasons) or leaving it alone (ecological reasons). The most common technique to remove sargassum from the beach front is to use mechanical equipment to rake the material and deposit it at the base of the dunes ( CONAWAY and WELLS, 2005; DUGAN et al., 2003; GHESKIERE et al., 2006; NORDSTROM et al., 2006) A study was initiated to determine the geomorphic and ecological impact of cleaning the sargassum off of the forebeach. Our first objective was to compare elevation changes at raked versus unraked beaches over two year period. Our second objective was to determine if raking the sargassum negatively impacts birds and fauna. Our third objective was to investigate the nutrients behind the beneficial effect of sargassum deposition upon the growth of common beach plants (WILLAMS and FEAGIN 2007). As local citizens often wash sargassum and then use it for fertilizer in their gardens, we wanted to additionally investigate the difference between washed and unwashed treatments of sargassum upon plant growth. Figure 1. Specimen of Sargassum spp., with picture of deposition in Galveston Island, June 2007. 4 Methods Elevation Changes As a continuation of the first year’s sampling (WILLAMS and FEAGIN 2007, WILLIAMS and FEAGIN 2008), we sampled for a second year, at three replicate sites on raked and unraked beaches on Galveston Island to determine if raking has an effect on beach elevation (Figure 2). Galveston Island is split into the east end and west end by a seawall that runs along the shoreline in the middle of the island. The island is accreting on the east while eroding on the west end (GIBEAUT et al., 2000). To account for the known variation between the accreting and eroding beaches, a blocking factor was established with two levels: East End versus West End. In order to test the effect of raking the sargassum upon elevation, there were two treatments: raked versus unraked. In general, the Galveston Island Park Board uses two types of equipment and techniques to clean the beach. The first technique is called “raking” or “grooming”. Raking refers to combing the beach with a beach rake that has spokes and is pulled behind a 3 point hitch tractor. This technique is used when there is only a small amount of wrack on the beach and is intended to mix up the sand and wrack in order to cover the wrack that is visible. Raking is typically done once a day during the tourist seasons from approximately March to September. The second technique is called “relocation” and is done using an articulated end loader. Relocation of the wrack is used when there are large amounts of wrack on the beach, in particular sargassum. On Galveston Island, none of the wrack is moved off of the beach, but instead it is relocated to the base of the dunes. Relocation may be followed by raking in order to create a smooth, aesthetic surface along the beach. Relocation of sargassum is typically done once a day when high levels of deposits are present on the beach. There were two sites randomly chosen within the possible locations for each treatment x block combination. On the East End, the two raked sites were in front of hotels, while the unraked sites were located in front of undeveloped land. On the West End, the raked beaches were in front of developed public parks, while the unraked sites were in front of undeveloped land. Within each replicate site, three transects were established relative to a permanent benchmark. Each transect was randomly located within the 50 meter wide site. Elevation was measured every three months from April 2006‐April 2007 using survey equipment (CST Berger PAL 26 level and tripod). Elevation was recorded at 5 meter intervals along the transect, from the dunes to the water line. The length of each transect varied from one sampling period to the next, as based on the width of the beach relative to the tide level. Elevation changes over the entire length of the study period were calculated as the difference between April 2006 and April 2008, but we also calculated each two years separately. Using Analysis of Variance (ANOVA), we tested the elevation changes in the transects in three ways: the overall average of the transect, the average of twenty meters from the waterline, and the average of twenty meters from the dunes. The three categories were assigned to capture the spatial heterogeneity of the beach from the water to the dunes. SPSS 14.0 (Release 14.0.1 18 Nov 2005) was used. This experimental design allowed us to test the difference between the raked versus unraked treatments, while balancing out the blocking factor variance. It also allowed us to capture the variation of each experimental unit, i.e. the replicate sites, as well as the variation within each site, i.e. where transects are sub‐samples of the site variation. 5 Figure 2. Study Site of East and West replicates on Galveston Island. Animal Sampling Sargassum samples were collected from the four unraked sites. Samples of 0.5 m2 were collected from the high tide wrack. Each sample was dried for 24 hrs and weighed to the nearest 0.1g. Dry weights samples ranged from 241g–1387g. Weights varied due to the height of the sargassum collected. Sargassum tufts were then rinsed with freshwater. Fauna retained on a 1 mm sieve were identified to lowest possible taxon. In determining the amount of epifaunal coverage, the algae were randomly sub‐
sampled in 1 gram increments. Percent coverage was estimated on each sub‐sample and averaged for the overall percent coverage. At each of the unraked replicate sites, observations of birds were taken for a duration of 15 minutes on a single day. Birds were observed for a 100 m study area between the waterline and the landward edge of the sargassum. On the east end sites, the study area was 17.0 m wide. On the west end sites, the study area was 12.5 m wide. The different widths are based on the variability in the amount of sargassum at the different locations. Counts of the species of birds and a description of the use of sargassum were recorded. 6 Nutrient Sampling Sargassum was collected from Galveston Island, TX in July 2007 and was transported in an identical method as the sargassum that was used for an earlier greenhouse study that showed that sargassum increased the growth of the dune perennial Panicum amarum, or beach panic grass (WILLIAMS and FEAGIN 2007). Unwashed versus washed sargassum samples were compared to determine potential differences in nutrient composition. As local citizens often wash sargassum and then use it for fertilizer in their gardens, we wanted to replicate this action for two reasons (1) to investigate the effect of this action in an applied sense, and (2) to remove sedimentary and salt accumulation deposits on the sargassum – for the purpose of isolating the sargassum versus sargassum plus dried seawater effect upon plant growth. Washed sargassum samples were rinsed with tap water and the ten 100 ml samples of tap water that had been rinsed through the sargassum was then collected. Each filtered sample of rinse water was analyzed separately. Tap water samples were analyzed as a control. No rinsing occurred for the unwashed sargassum samples. The sargassum samples were then dried for approximately 24 hours, crushed with a rolling pin and sieved to remove any sand grains. Next, the sargassum was passed through a steel electric "flail‐arm" Soil Grinder (Humbolt Mfg Co. Northridge IL 60706) and a Tecator Cyclotec 1093 Sample Mill and ground into a powder. The sargassum and water samples were analyzed for nitrogen (N), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg) and phosphorous (P). Additionally, the sargassum samples were analyzed for carbon (C). Analysis of N in water was conducted with flow injection spectrophotometer (AlpKem division of O.I Analytical, College Station, Texas, Model: #FS3000 with TKN ‐gas diffusion cartridge module). Analysis of N and C in the sargassum was conducted on the dried material by a flash dynamic combustion method using a furnace, separator, and detector manufactured by ThermoFinnigan (CarloErbaInstruments, Milano Italy, Model: Flash EA1112 carbon/nitrogen analyzer). For the analysis of K, Na, Ca, Mg and P, the sargassum samples were subject to a wet digestion procedure, using a solution of sulfuric acid, hydrogen peroxide, selenium, and lithium sulfate. The samples were slowly heated for approximately six hours until the material was completely digested (i.e. liquid was clear). Before being ready for analysis, the resulting liquid was cooled, diluted, cooled again and filtered. Analysis of K, Na, Ca and Mg for the digested sargassum and water samples were conducted with an atomic absorption flame spectrophotometer (Varian Inc., Model: SpectrAA 220 Fast Sequential with SIPPs pump sample auto diluting system). Analysis of phosphorous for the digested sargassum and water samples was conducted with the same flow injection spectrophotometer used for the N analysis with a total phosphorous cartridge module. T‐tests were used to analyze the difference between nutrients in the washed and unwashed sargassum and between the rinse water and the standard tap water. 7 Results Elevation Changes The statistical analysis indicate that there were no significant differences between Raked/Unraked or East/West sites (Table 1). The only exception occurred at the waterline for the elevation changes during the 2006‐2007 period. This change was explained in the original report as likely occurring due to daily events such as tides and wave action (WILLIAMS and FEAGIN 2007, WILLIAMS and FEAGIN 2008). There were no interaction effects for the 2006‐2007 period, however there were interaction effects between for the time periods 2006‐2008 and 2007‐2008. Therefore, we ran separate ANOVAs for East and West sites each of the three time periods. Table 1. Blocked ANOVA for All Three Time Periods 8 The separate East/West ANOVAs (Table 2) resulted in significant differences between the transects on each site. This indicates that our sampling method does pick up differences that occur. Many of the significant differences between replicates occurred on the East sites. This can be contributed to the greater length of the East sites, which results in a more variable profile from the dune to waterline. The West sites’ dune structures are less pronounced than the East sites. Overall, there were no significant results for Raked/Unraked sites for any of the ANOVAs for all three time periods. This additionally supports the original findings from 2006‐2007 that raking does not result in a significant elevation change on beaches. Table 2. Separate ANOVAs for East and West Sites for All Time Periods. Additionally, elevation data for all six sample dates were plotted for visual analysis. The data was plotted for each site separated by transects to identify visually if there were any major differences between replicates at each site over time (Figures 3 and 4). The data was also plotted for the three transects for each site separated by sampling dates (Figures 5 and 6). This allowed analysis of the overall fluctuations of each site over the two years. The visual analysis supported the statistical results as it showed that there were no obvious differences between elevation changes in raked and unraked locations. For most of the sites, visually, the elevation stayed at a constant height and slope. There were some exceptions which will be described individually below. 9 Figure 3. Plots of data for the East sites from April 2006 to May 2008. “Flag” represents the starting point in the dunes. Where the line reaches zero represents the waterline for that sample day. 10 Figure 4. Plots of data for the West sites from April 2006 to May 2008. “Flag” represents the starting point in the dunes. Where the line reaches zero represents the waterline for that sample day. 11 By Transects: On the East End (Figure 3), raked site “East Hotel 2” (Figure 3c) exhibited decreased elevation over all transects from April 2006 (red) to May 2008 (light blue). “East Unraked 2” (Figure 3d) exhibited overall increased elevations over all transects near the dunes. The peaks in elevation near the dunes are due to artificial dune formations from raking depositions. On the West End (Figure 4), raked site “West Pocket Park 3 (PP3)” (Figure 4a) and Unraked site “West Galveston Island State Park (GISP)” (Figure 4d) exhibited decreased elevation over all transects from April 2006 to May 2008. By Sampling Dates: On the East End (Figure 5), a public access path was built through the raked site “East Hotel 1” (Figure 5a) between April 2007 and May 2008. The access path consists of a flattened sand path with a rope fence. The path is about one foot to the east of transect 1. The movement of sand in order to create the pathway may be the cause of the unconventional elevation levels on this site in May 2008. “East Unraked 1” (Figure 5b) exhibited the greatest decrease in elevation in January 2007. Raked site “East Hotel 2” (Figure 5c) exhibited an obvious decreased elevation and a steeper slope over time. As mentioned before, the peaks in elevation near the dunes are due to artificial dune formations from raking depositions. “East Unraked 2” (Figure 5d) exhibited steeper slopes overtime at the water line, but only slight decreased elevation overall. The reason for the unconventional pattern in May 2008 is unknown. On the West End (Figure 6), raked “West PP3” (Figure 6a), which also had the greatest decrease in elevation in January 2007, exhibited steeper slopes overtime at the water line, but only slight decreased elevation overall. Of the two unraked sites, “West Unraked” (Figure 6b) exhibited a steep slope in May 2008 and “West GISP” exhibited a decrease in elevation in May 2008, with transect 3 having the biggest loss. 12 Figure 5. Elevation changes of the three transects on each of the East sites. 13 Figure 6. Elevation changes of the three transects on each of the West sites. 14 Animal Sampling Fauna found within fresh sargassum wrack can be denoted from three habitats: pelagic, beach‐endemic, and terrestrial. Pelagic fauna was the dominate category. Pelagic fauna was predominantly collected dead, while terrestrial and beach‐endemic taxa were alive. For non‐colonial fauna, abundance counts were done for each group (to lowest possible taxon) (Table 3). The primary contributor to biomass was the beach‐endemic amphipod Orchestia spp. This species is a detritivore that decomposes sargassum. To most people, this species is a ‘fly’. Numerically, the pelagic gastropod Litiopa melanostoma dominated. For the attached fauna, percent coverage was determined (Figure 7). The encrusting bryozoan, Membranipora, coated over half of the sargassum wrack’s outer surfaces. In addition to fauna, the following filamentous algae were found attached to the sargassum. Ceramium sp., Enteromorpha sp., and two unidentified Rhodophyta were represented. There were also various land and freshwater pulmonate gastropods noted on the west unraked sites. Table 3. Non‐Colonial Fauna Identification within sargassum on unraked beaches Non-colonial Fauna in
sargassum Wrack
Order
West
Unraked
1
West
Unraked
2
Classification
East
Unraked
1
East
Unraked
2
Total
Abundance
Litiopa melanostoma
1
0
5
55
61
Epitonium krebsii
0
0
0
5
5
fam. Vermetidae
0
0
0
1
1
fam. Arcidae
1
0
0
0
1
Anoplodactylus lentus
1
0
3
1
5
Arachnida
0
4
0
0
4
Insecta
2
3
2
0
7
megalops larvae
2
0
0
0
2
Orchestia spp.
0
18
0
0
18
Corophium sp.
0
0
1
0
1
fam. Haustoriidae
0
1
0
0
1
fam. Caprellidae
2
0
0
1
3
Paradella dianae
0
0
6
2
8
Ophiuroid juvenile
3
0
0
0
3
Gastropoda
Pycnogonida
Decapoda
Amphipoda
Isopoda
Echinodermata
15 Figure 7. Percent coverage of colonial fauna for the four unraked sites. Percent Cover of Colonial Fauna Species
80%
West Unraked 1
70%
West Unraked 2
East Unraked1
60%
East Unraked 2
Total Average
50%
40%
30%
20%
10%
0%
Membranipora sp.
Aglaophenia latecarinata
Obelia sp.
Species
Plumularia sp.
Figure 8. Observations of two bird species, (A) laughing gulls (Larus atricilla) and (B) a boat‐tailed grackle (Quiscalus quiscula), utilizing the waterline at unraked sites in Galveston Island on June 13, 2007. A B 16 There was a total of six species of birds that were observed using the sargassum at all of the unraked sites (Table 4). The most commonly seen species was the laughing gull (Larus atricilla, Figure 8). Most birds were sited at the water line, not necessarily interacting with the sargassum. However, there were thirteen individuals that did peck at the fresh wrack in the water or on the beach: one willet (Cataptrophorus semipalmatus), eleven laughing gulls (L. atricilla), and one boat‐tailed grackle (Quiscalus. major, Figure 3). Additionally, there were eight least terns (Sterna antillarum) and one boat‐
tailed grackle (Q. major) sited sitting in the sargassum. Table 4. Avian Observations Common Name Latin Name OBSERVATIONS* West Unraked 1 West Unraked 2 East Unraked 1 East Unraked 2 1
1
0 1 pecking fresh sargassum 8 sitting in sargassum 2 (pair)
0 0
willet Catoptrophorus semipalmatus least tern Sterna antillarum laughing gull Larus atricilla 11
0
11 pecking fresh sargassum 41
reddish egret Egretta rufescens 1
0
0 0
caspian tern Hydroprogne caspia 1
0
0 0
boat‐tailed grackle Quiscalus major 0
0
0 1
*All observations were taken for a span of 15 minutes in the afternoon of June 13, 2007. Unless stated, birds
were at the waterline but not specifically interacting with the sargassum 17 Nutrient Sampling Washing the sargassum increased the proportion of C, but significantly depleted N, Na and P (Figure 9). For N, Na, Ca and Mg equal variances were assumed. For C, K, and P equal variances were not assumed. There was a significant change in C (p‐value = 0.009), N (p‐value = 0.003), Na (p‐value = 0.002) and P (p‐
value = 0.021) between unwashed and washed sargassum. The rinsed water showed a significant increase in all the nutrients analyzed (N, K, Na, Ca, Mg and P) when compared to the standard tap water (p‐value ≤ 0.001). Figure 9. Nutrient Sampling Results. 5
parts per million (ppm)
4
3
2
1
0
N
K
Na
Ca
Nutrients
Mg
Phos
unwashed
washed
18 Discussion The first part of the study indicated that during a two‐year time period, beach raking did not cause a significant change in elevation as compared to unraked beaches. The second part of the study supports other research (GHESKIERE et al., 2006; ORR et al., 2005; ROBERTS and POORE 2005; RYLAND 1974; TANAKA and FOSCA 2003) showing that sargassum can provide habitat value for birds and a variety of pelagic, beach endemic and terrestrial fauna. However, though a few birds did land on the older deposits of sargassum at the high tide line, our observations indicated that avians mainly used the fresh sargassum that was located at the waterline. This indicates that the birds may depend on fresh influxes of sargassum more than the older deposits of sargassum at the high tide line. The unattached epifauna that washes ashore with sargassum may be targeted as a food source by beach megafauna (i.e. shorebirds, land crabs). Small beach‐endemic and terrestrial invertebrates could target the unattached sargassum biota, but also may feed on the more common attached colonial and non‐colonial sargassum fauna. However, from our observations, fauna was not observed using the sargassum at the high tide line to scavenge. Since fresh influxes of sargassum would still be deposited on the beach during the 24 hour period between raking, birds may not be impacted by daily beach raking methods. For bird usage, it is thus important to draw the distinction between fresh sargassum and older sargassum. Management activities that remove fresh sargassum will certainly impact shorebirds. However, management activities that remove older deposits should have a much lower impact, based upon the data that we have collected. Our work also suggests a reason for the use of predominantly fresh deposits – the pelagic fauna dies as it dries out, losing its appeal for birds. We thus recommend that raking not disturb fresh deposits and that tractors should not be driven at the immediate waterline. The third part of the study explains why sargassum has been found to enhance plant growth of Pancium amarum (beach panic grass) during greenhouse studies, in earlier research (WILLAMS and FEAGIN 2007). In this earlier research, we found that after washing the sargassum and then depositing it, the beneficial effect was lessened. The results in the present study show that N, Na and P were significantly depleted from the sargassum when it was washed. Dune plants are typically limited by N, K and P availability, and Panicum amarum is particularly sensitive to N and P (HESTER and MENDELSSOHN, 1990). Thus, results suggest that the primary mechanism of enhanced Panicum amarum growth in our earlier experiments is from the additional subsidy of N and P. By washing the sargassum, some portion of this N and P subsidy is mobilized and lost. We suspect that Na may actually enhance growth of Panicum amarum as well. Dune plants grow in an environment where they encounter salt‐spray and salt water inundation and they are known to be salt tolerant, but future work needs to be conducted as to whether Na addition may be beneficial up to a given threshold. Other nutrients (K, Mg, and Ca) and micronutrients in the sargassum may enhance plant growth as well, but they are unlikely to be responsible for the significant differences between the washed and unwashed sargassum, nor as the primary mechanism affecting Panicum amarum growth in general (HESTER and MENDELSSOHN, 1990). Future work could focus upon specific nutrient additions where each nutrient type is singularly applied to Panicum amarum. Further research would need to be done to see if sargassum helps the growth of other types of dune plants besides Panicum amarum. It may be possible to capitalize on the benefits of sargassum by utilizing it as a fertilizer in dune restoration projects. There may be a difference in the nutrients needed in garden varieties of plants from dune plants that should be explored to see if sargassum could also be used as a commercial fertilizer. 19 Future studies over extended time periods (such as over 5‐10 years) would also be beneficial in determining long term impacts of beach raking. However, if other geomorphic processes dwarf the impact of raking upon elevation change on a 1‐2 year time scale, it is unlikely that longer time scales will resolve an impact. At longer time scales, events such as hurricanes could mask any effect by moving large amounts of sediment. The only scenario in which a longer time scale could better resolve the impact upon elevation, is one in which the beach profile response is episodic in response to specific conditions; thus longer‐term sampling could pick up these events. Longer sampling periods would be useful for the avian portion of the study. This portion of the study was limited to one short afternoon time period. Birds feed at different times of the day. Future studies should include morning, afternoon, and night observations to determine if birds use different niches of the wrack or if wrack is used in other ways such as nesting, camouflage, or social aggregation. In previous studies (WILLIAMS & FEAGIN 2007, WILLIAMS & FEAGIN 2008), we have found evidence that when the raked sargassum is placed along the dunes, it seems to enhance dune plant growth of the species beach panicum (Panicum amarum). However, additional research is needed to determine if moving the sargassum from the niche of the high tide line to the base of the dunes would impact specific fauna. Also, further sampling may provide a more extensive list of possible native fauna related to the sargassum wrack. More research is needed into the effect on running tractors across the beach upon crabs, and into the economic impacts of sargassum upon recreational preferences and beach tourism. 20 Conclusion This study indicated that beach raking did not significantly change the elevation of raked beaches, when compared with unraked beaches, over a two year time period. The study also indicated that although there are many pelagic species found in the sargassum, most pelagic fauna are dead and not using the sargassum for habitat once it is on the beach. Raking and depositing the sargassum into new locations likely affects the terrestrial Orchestia spp. the most negatively (a detritivore ‘fly’). We have also found that raking sargassum and then depositing it at the base of the dunes helps dune vegetation grow through the input of primarily nitrogen and phosphorus. Thus, beach raking may be an adequate method for removing sargassum. 21 References COLOMBINI, I. and CHELAZZI, L., 2003. 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