Sea level rise and the significance of marginal beaches for horseshoe crab spawning Mark L. Botton 1 and Robert E. Loveland 2 1 Fordham University, Dept. Natural Sciences and Environmental Science Program 2 Rutgers Univ., School of Environ. and Biol. Sci., Dept. Ecol. Evol. Nat. Resources
Horseshoe crabs use of intertidal habitats is ancient
Causes of Relative Sea Level Change (modified from http://www.meteor.iastate.edu/gcp/sealevel/images/11.gif)
Annual averages of the global mean sea level based on reconstructed sea level fields since 1870 (red), tide gauge measurements since 1950 (blue) and satellite altimetry since 1992 (black). Units are in mm relative to the average for 1961 to 1990 (source: IPCC, 2007).
Documenting relative sea level rise NOAA/NOS National Water Level Observation Network 175 long-term, continuously operating water-level stations throughout the USA http://tidesandcurrents.noaa.gov/nwlon.html
Range of Limulus polyphemus in North America Canada Maine USA Atlantic Ocean Mexico Gulf of Mexico Yucatan
USA Coast New England
USA Coast Mid-Atlantic
USA Coast South Atlantic
USA Coast Gulf of Mexico
Measured Rates of Sea Level Rise on U.S. Coast (source: NOAA) Region Station Rate (cm per 100 yr) New England Portland, ME 18.3 Woods Hole, MA 26.2 Newport, RI 25.9 New London, CT 22.6 Middle Atlantic New York City, NY 27.7 Sandy Hook, NJ 39.0 Atlantic City, NJ 39.9 Cape May, NJ 40.5 Chesapeake Bay (Annapolis, MD) 34.4 Wilmington, NC 20.7
Measured Rates of Sea Level Rise on U.S. Coast (source: NOAA) Region Station Rate (cm per 100 yr) South Atlantic Charleston, SC 31.5 Fort Pulaski, GA 29.8 Fernandina Beach, FL 20.2 Gulf of Mexico Pensacola, FL 21.0 Grand Isle, LA 92.4 Galveston, TX 63.9
Sea level rise has been documented in many areas of Asia, where Tachypleus tridentatus, T. gigas, and Carcinoscorpius rotundicauda are found
Seto Inland Sea, Japan
Hong Kong Wong et al. Hong Kong Meteorol. Soc. Bull. 13: 24-40
The Bruun Rule: Shoreline retreat >> vertical rise in sea level For typical beach slopes and sediments, shoreline retreat (R) is approximately 50 to 100 times vertical sea level rise (S) (from FitzGerald et al. (2008) Ann. Rev. Earth Planet. Sci. 36: 601-647)
Paleogeographic reconstructions of Delaware Bay [modified from Knebel et al. (1988) Mar. Geol. 83: 115-133]
Delaware Bay 16,000-18,00 Yrs BP
Delaware Bay 12,000-15,000 Years BP
Delaware Bay 10,000-11,000 Yrs BP Sandy beach
Delaware Bay 7,000-8,000 Yrs BP Sandy beach
Delaware Bay 5,000-6,000 Yrs BP Sandy beach
Rapid erosion along the Delaware Bay shoreline Norbury s Landing, Cape May County, NJ May, 1990
July, 2001
January, 2007
Long ago, before humans developed the coastline, beaches migrated landward during times of sea level rise No doubt, horseshoe crabs tracked the movements of these critical habitats
So, should we be concerned today about the effects of sea level rise on horseshoe crabs?
We now have various shore protection structures along the coastline Sea Breeze, NJ Kasaoka, Japan Fortescue, NJ Raybins Beach, NJ
Erosion of open bay beaches in Delaware Bay is exposing ancient salt marsh peat, making them less suitable for HSC spawning Botton et al. (1988) Mar. Biol. 99: 325-332
Consequences of sea level rise for horseshoe crabs Loss of spawning habitat from erosion and shoreline armoring Exposure of unsuitable sediments (e.g. peat) Horseshoe crabs may get squeezed into smaller areas of optimal beach, or into more marginal habitats
Examples of marginal habitats Hardened shorelines Tidal creek areas Small patches of sand within/above marshes
Egg density as an index of habitat suitability
Developmental success: comparing optimal and marginal habitats
Delaware Bay East Point New Jersey
East Point (black line = shoreline position in 1930 s)
East Point Lighthouse Beach
East Point Overwash Study Area
Delaware Bay Kimbles Beach New Jersey
Kimbles Beach Tidal creek Sand Bar Open Beach
Kimbles Open Beach
Kimbles Tidal Creek
Kimbles Sand Bar
Delaware Bay Pierces Pt. New Jersey
Pierces Point Open beach Bulkheads
Marginal habitats in Delaware Bay, NJ Marginal habitats such as bulkheaded, overwash, or erosional beaches, will still attract spawning horseshoe crabs provided these areas have well drained, aerobic, deep sands Based on egg density, sand bars and tidal creeks appear to be hot spots for spawning horseshoe crabs (and important shorebird habitat)
Jamaica Bay: An urban estuary in New York City Extensive bulkheading Limited freshwater inputs and high pollutant loading Very densely populated
Dead Horse Bay
Dubos Point
Spring Creek
Brant Point
Bayswater Sand spit Sea wall
Does water quality affect the developmental success of horseshoe crabs in Jamaica Bay? Photo: Botton et al. (2010) Curr. Zool. 56: 550-562
Embryos collected from JB were cultured in water from Jamaica Bay and Delaware Bay in Spring, 2000 Total Number of Embryos Examined: JB (2,428); DB (1,466) Percent Normal Embryos: JB (99.05%); DB (98.84%) Botton et al. (2006) Estuaries & Coasts 29: 820-830
Marginal habitats in Jamaica Bay The only game in town! Sediment texture is key The JB population is probably limited by the availability of spawning habitat rather than by water quality Optimal spawning habitat in JB is very scarce, which may make it a useful system to compare with many Asian horseshoe crab populations
Summary Changes in relative sea level are nothing new to horseshoe crabs, given their long geological history There is convincing evidence for recent and ongoing sea level rise Shorelines are dynamic habitats. During past sea level rises, spawning beaches were able to migrate landward
Humans and horseshoe crabs are both estuarinedependent species Rises in relative sea level causes erosion of beach houses, roads, etc. Beach stabilization to protect coastal property affects the landward migration of the shoreline during times of sea level rise Horseshoe crab populations are impacted by sea level rise, largely due to the loss of optimal spawning habitat
Horseshoe crabs are adept at locating and depositing eggs in small patches of suitable habitat. The developmental success of Limulus eggs laid in marginal habitats in Delaware Bay and Jamaica Bay is high, but similar studies of the three Asian species should be conducted.
Implications for Horseshoe Crab Conservation Today Optimal spawning habitats are becoming scarcer due to sea level rise and human disturbance of the shoreline The survival of horseshoe crab populations may depend on their ability to exploit patches of marginal spawning habitats
A Longer View of the Importance of Marginal Habitats 2011 16,000-18,000 Years BP Knebel et al. (1988)
There were no long stretches of sandy beaches in Delaware Bay during the last Ice Age Where did horseshoe crabs lay their eggs?
Habitats such as tidal creeks and sand bars may be important refugia for horseshoe crab populations when sandy beaches are limited in size
Some additional speculations Smaller, discrete spawning populations with adults possibly remaining closer to the spawning beaches rather than dispersing widely along the continental shelf (closer to the present-day New England model of Limulus populations than to the Middle Atlantic model Retention near spawning beaches facilitated by larval behaviors possible implications for genetic structure of populations
Special Thanks Dr. Carl Shuster Principal research support from NJ Sea Grant, PSE&G, National Park Service, NJ Audubon Society, NJ DEP, and The Nature Conservancy