Beach Compaction and the Impacts of Tilling on Nesting Sea Turtles and Foraging Shorebirds
Researchers are investigating beach compaction, the incubation environment for turtle nests, and shorebird nest abundance at beaches throughout the southeastern United States to better understand the impacts of beach compaction to nesting turtles and foraging seabirds.
The Science Issue and Relevance: Sandy beaches are one of the nation’s most valuable natural resources. Because of this, they are monitored, maintained, and rebuilt on a regular basis. Beach nourishment, which involves placing new sand on eroding beaches, has become the standard method of beach management over the past three decades. Because sea turtles nest on sandy beaches, altering the sandy beach environment through nourishment has obvious implications for sea turtle nesting and as a result, the impacts of beach nourishment to sea turtle nesting have been frequently examined. Several studies have reported that nourishment lowers reproductive success, which has been attributed in large part to increased sand compaction that makes it more difficult for adult female turtles to dig nests and for hatchlings to emerge from those nests. Because of this, regulators now require nourishment projects to till the beach for three years post-nourishment to loosen the sediment and reduce compaction.
While tilling is a generally accepted method of reducing compaction, the benefit of this action to sea turtle nesting remains unclear. Additionally, tilling may have negative impacts to shorebirds foraging on those same sandy beaches. Tilling may alter the wrack-line which provides important foraging habitat for shorebirds. Wrack-associated animals, such as amphipods, isopods, and insects, represent primary prey for higher trophic level species including shorebirds, and animal communities are significantly reduced in species richness, abundance, and biomass by wrack disturbance. These impacts are particularly alarming for species listed under the Endangered Species Act that rely on the wrack for foraging such as snowy and piping plovers. Destruction of the wrack not only impacts shorebirds but also other coastal organisms such as beach mice, seabirds (e.g., least terns), and plants. These impacts can be far-reaching; loss of plants may impact dune development which may increase erosion and reduce habitat availability for coastal species. It has been suggested that tilling can be conducted above the wrack line to reduce impacts to foraging birds; however, this may alternatively destroy sea turtle nests, impact roosting habitat for plovers, and affect reestablishing plants such as sea oats which occur higher on the beach.
Some of the uncertainty surrounding compaction impacts to nesting turtles derives from the inaccuracy of equipment used to measure beach compaction, namely the static cone penetrometer. Recently, more advanced penetrometers, called dual-mass dynamic cone penetrometers, have been shown to more accurately document compaction on nesting beaches. Use of this advanced technology would enable U.S. Fish and Wildlife Service to re-examine the tilling requirement based on empirical data gathered on southeastern U.S. beaches. More data are needed to better understand the impacts of beach compaction to nesting turtles and to assess the need for tilling following sand placement. We are addressing this issue by investigating beach compaction, the incubation environment for turtle nests, and shorebird nest abundance and distribution pre- and post-nourishment at beaches throughout the southeastern United States.
Methodology for Addressing the Issue:
1. Archived data
We are gathering historic sea turtle and shorebird nesting data, sea turtle nesting success, including hatching success and incubation length, on select beaches across northwest Florida that have undergone beach nourishment and tilling. Using these data, we are comparing nest abundance and success pre-nourishment to post-nourishment, and to data from an adjacent, natural beach.
2. Compaction measurements
We are measuring compaction with a dual mass dynamic cone penetrometer at 12 sites in St. Joseph State Park, including areas previously nourished in 2019 and areas never nourished (e.g., northern portions of the State Park). At these 12 sites, compaction data are being gathered monthly from May 1 through October. Additionally, compaction readings are being gathered at beaches that are scheduled for nourishment. These readings will be collected:
A. immediately prior to and immediately after (but prior to tilling) beach nourishment
B. immediately after, 6 months after, and 1-year post tilling
C. immediately adjacent to and within the boundaries of false crawl body pits (body pits that have been abandoned) and nesting body pits (avoiding the clutch) along the St. Joseph Peninsula.
3. Sand temperatures
Sand temperatures were gathered on the St. Joseph Peninsula from 1998-2003 (see Lamont and Johnson 2020). To update those data, we have a long-term monitoring program across nesting beaches in the Gulf of Mexico, including Kemp’s ridley sites in northern Mexico. Temperature loggers are buried along paired transects at three depths corresponding to the bottom, middle, and top of a sea turtle clutch. Loggers are deployed at the base of the dunes and mid-beach and record hourly data from May 1 until October 31 each year.
Future Steps: Updating current sand temperature loggers to include water sensors will provide information on nest inundation. Inundation may cause direct mortality to incubating eggs and plays a large role in driving sand temperatures.
Researchers are investigating beach compaction, the incubation environment for turtle nests, and shorebird nest abundance at beaches throughout the southeastern United States to better understand the impacts of beach compaction to nesting turtles and foraging seabirds.
The Science Issue and Relevance: Sandy beaches are one of the nation’s most valuable natural resources. Because of this, they are monitored, maintained, and rebuilt on a regular basis. Beach nourishment, which involves placing new sand on eroding beaches, has become the standard method of beach management over the past three decades. Because sea turtles nest on sandy beaches, altering the sandy beach environment through nourishment has obvious implications for sea turtle nesting and as a result, the impacts of beach nourishment to sea turtle nesting have been frequently examined. Several studies have reported that nourishment lowers reproductive success, which has been attributed in large part to increased sand compaction that makes it more difficult for adult female turtles to dig nests and for hatchlings to emerge from those nests. Because of this, regulators now require nourishment projects to till the beach for three years post-nourishment to loosen the sediment and reduce compaction.
While tilling is a generally accepted method of reducing compaction, the benefit of this action to sea turtle nesting remains unclear. Additionally, tilling may have negative impacts to shorebirds foraging on those same sandy beaches. Tilling may alter the wrack-line which provides important foraging habitat for shorebirds. Wrack-associated animals, such as amphipods, isopods, and insects, represent primary prey for higher trophic level species including shorebirds, and animal communities are significantly reduced in species richness, abundance, and biomass by wrack disturbance. These impacts are particularly alarming for species listed under the Endangered Species Act that rely on the wrack for foraging such as snowy and piping plovers. Destruction of the wrack not only impacts shorebirds but also other coastal organisms such as beach mice, seabirds (e.g., least terns), and plants. These impacts can be far-reaching; loss of plants may impact dune development which may increase erosion and reduce habitat availability for coastal species. It has been suggested that tilling can be conducted above the wrack line to reduce impacts to foraging birds; however, this may alternatively destroy sea turtle nests, impact roosting habitat for plovers, and affect reestablishing plants such as sea oats which occur higher on the beach.
Some of the uncertainty surrounding compaction impacts to nesting turtles derives from the inaccuracy of equipment used to measure beach compaction, namely the static cone penetrometer. Recently, more advanced penetrometers, called dual-mass dynamic cone penetrometers, have been shown to more accurately document compaction on nesting beaches. Use of this advanced technology would enable U.S. Fish and Wildlife Service to re-examine the tilling requirement based on empirical data gathered on southeastern U.S. beaches. More data are needed to better understand the impacts of beach compaction to nesting turtles and to assess the need for tilling following sand placement. We are addressing this issue by investigating beach compaction, the incubation environment for turtle nests, and shorebird nest abundance and distribution pre- and post-nourishment at beaches throughout the southeastern United States.
Methodology for Addressing the Issue:
1. Archived data
We are gathering historic sea turtle and shorebird nesting data, sea turtle nesting success, including hatching success and incubation length, on select beaches across northwest Florida that have undergone beach nourishment and tilling. Using these data, we are comparing nest abundance and success pre-nourishment to post-nourishment, and to data from an adjacent, natural beach.
2. Compaction measurements
We are measuring compaction with a dual mass dynamic cone penetrometer at 12 sites in St. Joseph State Park, including areas previously nourished in 2019 and areas never nourished (e.g., northern portions of the State Park). At these 12 sites, compaction data are being gathered monthly from May 1 through October. Additionally, compaction readings are being gathered at beaches that are scheduled for nourishment. These readings will be collected:
A. immediately prior to and immediately after (but prior to tilling) beach nourishment
B. immediately after, 6 months after, and 1-year post tilling
C. immediately adjacent to and within the boundaries of false crawl body pits (body pits that have been abandoned) and nesting body pits (avoiding the clutch) along the St. Joseph Peninsula.
3. Sand temperatures
Sand temperatures were gathered on the St. Joseph Peninsula from 1998-2003 (see Lamont and Johnson 2020). To update those data, we have a long-term monitoring program across nesting beaches in the Gulf of Mexico, including Kemp’s ridley sites in northern Mexico. Temperature loggers are buried along paired transects at three depths corresponding to the bottom, middle, and top of a sea turtle clutch. Loggers are deployed at the base of the dunes and mid-beach and record hourly data from May 1 until October 31 each year.
Future Steps: Updating current sand temperature loggers to include water sensors will provide information on nest inundation. Inundation may cause direct mortality to incubating eggs and plays a large role in driving sand temperatures.