Christina Pruett and Mark Stolt

1. Development of Subaqueous Soil Interpretations: Eelgrass Restoration and Carbon Storage Christina Pruett and Mark Stolt University of Rhode Island, Kingston, RI 02881; contact: cpruett.p@gmail.com INTRODUCTION Estuarine subaqueous soils are permanently flooded soils that occur immediately below the intertidal zone to water depths of generally <2.5 m at extreme low tide in protected coves, bays, inlets, and in back-barrier coastal lagoons (Bradley and Stolt, 2003). Subaqueous soil mapping projects are underway along the Gulf and Atlantic coasts to provide an inventory of shallow subtidal habitats for management and development purposes similar to that which is available for terrestrial soils. Although mapping protocols for developing subaqueous soil surveys are well defined, the corresponding interpretations are lacking. A necessary component of terrestrial soil surveys are the land use interpretations that provide management information for each soil type. Possible interpretations of subtidal soils include use and management decisions related to dredging activities, mooring field construction, coastal hazards, shellfish production, and eelgrass restoration sites. Eelgrass (Zostera marina)is an important species of rooted vegetation in New England coastal ecosystems (Fig 2). In this study we explore two subaqueous soil interpretation tools: eelgrass restoration and carbon storage. OBJECTIVES 1. Test if certain soil-landscape units are better suited for eelgrass restoration 2. Develop an understanding of the role of subaqueous soils in the storage and sequestration of soil carbon on a landscape and regional scale. RESULTS Eelgrass Interpretation Pooled transect data from all three ponds showed that Flood Tidal Delta Slope and Lagoon Bottom soil-landscape units have significantly higher mean percent eelgrass cover than Washover Fan Flat, Washover Fan Slope and Flood Tidal Delta Flat landscape units (Figure 6). Although there is a higher percentage of eelgrass cover within the LB unit, transplants had lower survival rates within the LB unit in Potter Pond (Figure 7). Lagoon Bottom soil-landscape units are generally low-energy basins that are characterized by a higher percentages of silts, clays, and organic matter than sandier units (Table 1). Finer textured and more organic rich soils often correlate with elevated sulfide levels (sulfide data has not been completed). Zostera marina has been shown to devote more resources to leaf elongation rather than root production under periods of stress (Bintz et al., 2003 and Wicks et al. 2009). Although root biomass data is yet to be processed, leaf lengths were shown to be significantly longer in LB units in Potter Pond suggesting the LB transplants were stressed (Figure 8).Sources of stress to Zostera include low levels of light attenuation and high levels of soil organic carbon and sulfides. In addition, disproportionate shoot:root ratios can also result in plants being easily separated from the soil surface, especially in the high organic and silty soils as in a LB soil-landscape unit. Carbon Storage Interpretation Soil organic carbon contents vary with depth and among soil-landscape unit in subaqueous soils (Figure 9). Sandy textured-higher energy units, such as the Flood Tidal Delta Flat and Washover Fan Flat, typically have less than 0.6% SOC throughout the profile with the exception of buried A horizons. Low-energy landscape units such as the Lagoon Bottom and Mainland Cove units are basins that often have high organic carbon contents (4 to 6%, Figure 9). SOC pools in subaqueous soils are as large or in some cases larger than that found in subaerial soils (Table 2). Subaqueous soils have largely been overlooked as significant carbon sinks yet this table shows that they may be as important carbon sinks as some subaerial soils. The Flood Tidal Delta Flat SOC profile (Figure 9) illustrates that buried subaerial soils can contribute to the high SOC pools found within subaqueous soils. Buried organic paleosols are often encountered when sampling subaqueous soils. Although these relict soils contribute to the significance of subaqueous SOC pools, subaqueous soil types found in MC and LB units provide a great amount of the carbon storage within these soils (Figure 9). Our future work will analyze organic carbon distribution within estuaries in Rhode Island and discuss the importance of RI subaqueous soils as carbon storage units. Fig 5. c c b # Surviving Shoots b a Fig 6. Mean percent eelgrass cover per soil-landscape unit in Ninigret, Quonochontaug and Potter Ponds for the summers of 2008 and 2009. Means with different letters are significantly different (P ≤ 0.05). Fig 7. Survival of eelgrass shoots in Zostera transplant plots in Potter Pond. Standard error bars shown. Success rate of eelgrass transplants over 50% are considered good (Short, 2002) Fig 5. Vibracoring Method: An aluminum tube is vibrated into the soil to a depth of at least 1 meter. b Fig 1a. a a Fig 2. Fig 3. Fig 4. Table 1. Soil organic carbon content and particle size distribution for the top 25 cm of soil profile at Zostera study sites within Potter Pond. Fig 8. Differences in leaf lengths between soil-landscape units in Potter Pond. Means with different letters are significantly different (P ≤ 0.05). Standard deviation bars shown. Figures 2 to 4: Zostera marina. Tying Zostera shoots to TERF frames. TERF frames in situ. METHODS Eelgrass Interpretation During the summers of 2008 and 2009 eelgrass distribution was assessed within soil-landscape units in three Rhode Island coastal lagoons (Ninigret, Quonochontaug and Potter Ponds) (Figure 1). The point intercept transect method (Bradley and Stolt, 2006) was used to determine percent cover on Washover Fan Flat (WFF), Washover Fan Slope (WFS), Flood Tidal Delta Flat (FTDF), Flood Tidal Delta Slope (FTDS) and Lagoon Bottom (LB) soil-landscape units. Three replicate transects were performed in each landscape unit in 2008 and six replicate transects were performed in 2009 for a total of nine transects per landscape unit. Water depths and secchi depths were taken at each site to ensure habitable conditions for eelgrass. Surficial soil samples were taken at each site for soil characterization. To relate eelgrass distribution to eelgrass restoration success, experimental eelgrass transplants and growth rate experiments were deployed in three landscape units in Potter Pond in 2009. Potter Pond was chosen as the study site for this part of the study based on current eelgrass abundance and health. We transplanted eelgrass shoots according to the TERF™ transplant method (Short, 2002). In this method, 50 eelgrass shoots are tied to a biodegradable frame that is anchored to the soil surface. Transplant survival was determined monthly. Growth measurements of eelgrass were performed according to the plastochrone method (Duarte and Short, 2001). In this method a reference mark is made within the sheath of the eelgrass shoot with a syringe. Eelgrass is harvested after two to three weeks and leaf length, number of new leaves produced, and overall growth rates are measured. Carbon Storage Interpretation A vibracorer or McCauley peat sampler was used to collect soil cores for organic carbon analyses depending on bottom type (Figure 5) (Lanesky et al., 1979). Vibracore samples were collected in aluminum tubes and kept refrigerated at 4˚C until opened for description and analysis. Soil cores are cut lengthwise and described according to the Soil Survey Manual (Figure 10) (Soil Survey Division Staff, 1993). After each horizon has been delineated and described, subsamples are collected from each horizon for bulk density, loss on ignition, and soil organic carbon (SOC) content. Bulk density was determined using a syringe column to sample a fixed volume (10 cm3) of soil per horizon. Soil organic carbon was estimated after determining the loss on ignition at 550 degrees C combustion in a muffle furnace. Soil organic carbon content was estimated as 50% of the LOI. Soil organic carbon (SOC) pools per horizon were determined by the product of horizon thickness, horizon bulk density and SOC content (Jespersen and Osher, 2007; Veldkamp, 1994). Soil organic carbon pools for each profile were calculated as the sum of all horizons in the profile to a depth of 1 m. • CONCLUSIONS AND FUTURE WORK • Lagoon Bottom soil-landscape units in Rhode Island have the highest percent eelgrass cover of the landscape units studied. However, these units may not ideal for eelgrass restoration projects due to soil properties such as high amounts of SOC that may cause sulfide toxicity in eelgrass vegetation. Flood Tidal Delta Slope and Washover Fan Slope are better candidates for eelgrass restoration. • SOC generally decreases throughout subaqueous soil profiles unless a buried subaerial soil is present within the profile. SOC pools are highest in basin units such as Mainland Cove and Lagoon Bottom and lowest in sandier units such as Washover Fan Flat and Flood Tidal Delta Flat. • SOC pools in subaqueous soils are as large or in some cases larger than in terrestrial soils (Table 2). • Future work will include a comprehensive examination of SOC distribution within Quonochontaug, Ninigret and Pt. Judith Ponds. In addition, heavy metal analysis is ongoing to determine heavy metal concentrations and distributions in Rhode Island estuarine soils. Soil Profile Depth (cm) * * Fig 1a. Locus map of coastal lagoons that will be studied within Rhode Island. Washover Fan Flat Flood Tidal Delta Flat Lagoon Bottom Mainland Cove Fig 9. Soil organic carbon (SOC) profiles within different soil-landscape units. *Represents buried A horizon. Fig 1b. LB References: Bradley, M.P., and M.H. Stolt. 2006. Landscape-level seagrass-sediment relations in a coastal lagoon. Aquatic Botany 84:121-128. Bradley, M.P., and M.H. Stolt. 2003. Subaqueous soil-landscape relationships in a Rhode Island estuary. Soil Science Society of America Journal 67:1487-1495. Bintz, J.C., S.W. Nixon, B.A. Buckley and S.L. Granger. 2003. Impacts of temperature and nutrients on coastal lagoon plant communities. Estuaries 26:765-776 Jespersen, J.L. and L.J. Osher. 2007. Carbon storage in the soils of a mesotidal Gulf of Maine Estuary. Soil Science Society of America Journal 71:372- 379. Lanesky, D.E., B.W. Logan, R.G. Brown and A.C. Hine. 1979. A new approach to portable vibracoring underwater and on land. Journal of Sedimentary Petrology 49: 654-657. Short, F.T. and C.M. Duarte. 2001. Chapter 8: Methods for the measurement of seagrass growth and production. In: Short, F.T. and R.G. Coles. 2001. Global Seagrass Research Methods. Elsevier Science B.V. Amsterdam, The Netherlands. Short, F.T. et al. 2002. A Manual for Community Based Eelgrass Restoration. Jackson Estuarine Laboratory. Durham, NH. Soil Survey Laboratory Staff. 1993. Soil survey manual. Agric. Handbook No. 18, USDA-NRCS, U.S. Gov. Print Office, Washington, D.C. Soil Survey Staff. 1999. Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. 2nd edition. USDA-NRCS, Agriculture Handb. 436. U.S. Gov. Print Office, Washington, D.C. Veldkamp, E. 1994. Organic carbon turnover in three tropical soils under pasture after deforestation. Soil Science Society of America Journal 58: 175- 180. Wicks, E.C., E.W. Koch, J.M. O’Neil and K. Elliston. 2009. Effects of sediment organic content and hydrodynamic conditions on the growth and distribution of Zostera marina. Marine Ecology Progress Series 378: 71-80. FTDS FTDF WFF Fig 1b. Example of a soil-landscape unit map in Quonochontaug Pond (QP). Maps are produced based on bathymetric maps and field surveys of the area. Soil landscape units shown are Flood Tidal Delta Flat (FTDF), Flood Tidal Delta Slope (FTDS), Washover Fan Flat (WFF) and Lagoon Bottom (LB). Fig 10b Fig 10c Fig 10a Fig 10.Examples of vibracore samples. (a) Lagoon Bottom (b) Flood tidal delta flat and (c) Mainland Cove with horizons delineated The authors would like to thank the USDA – Natural Resource Conservation Service, NOAA and RI Agricultural Experiment Station for funding support. Penelope Pooler of the National Park Service provided statistical counseling and data analysis. Thanks to Maggie Payne, Alex Salisbury, Matt Ricker, Marissa Theve, Ed Talley, Mike Bradley and Jim Turenne for help in the lab and in the field. Table 2. Comparison of mean soil organic carbon pools for subaqueous and subaerial soils.