Chaetognaths (Phylum: Chaetognatha) are one of the most abundant phyla of zooplankton worldwide and play an important role in marine trophic interactions. Although the role of chaetognaths in global ecosystems is well understood, the spatial variation and environmental drivers of estuarine chaetognath populations is poorly understood. To provide the first known record of chaetognath species composition in a coastal estuary in the south-eastern USA, chaetognaths were identified and quantified from zooplankton samples collected on a monthly basis in 2019 and 2020 from North Inlet Estuary in South Carolina. Parasagitta tenuis was the most abundant species of the five found, making up 33% of total abundance. The egg presence of these chaetognaths was further analyzed to gauge reproductive cycles. Abundance and egg presence were compared with surface and bottom measurements of temperature, salinity, and dissolved oxygen levels to determine the driving abiotic factors behind chaetognath’s seasonal variability and reproductive cycles. Temperature, salinity, and dissolved oxygen all had low (r < ± 0.29), non-significant correlations with abundance. Chaetognath egg production was most significantly associated with dissolved oxygen (p < 0.001) and seasonal changes in temperature (p < 0.001). Our initial findings indicate the continued abundance of chaetognath in a local estuary is dependent on abiotic factors that are strongly influenced by a changing climate. 

Zooplankton samples were collected during one tow between 1000 and 14000 hours each month during outgoing mid-tide from November 2019 to November 2020. Tows were carried out horizontally, 20 meters from the marsh bank, parallel to the shore, and in the direction of the ebbing tide. Each tow lasted roughly 5 minutes and covered 200 meters. The tows were conducted aboard a boat with an epibenthic sled equipped with a 365-µm mesh net, a mouth opening of 50 cm wide by 35 cm high, and with a General Oceanics flowmeter attached to the net’s mouth. During the tow, the net mouth was just above the bottom, ~1.5m below the surface water. Surface (~0.5m below surface water) and bottom (~1.5m below surface water) salinity, temperature, and dissolved oxygen were measured from an anchored boat using a YSI Pro 2030 Sonde.

Upon collection, samples were immediately preserved in 4% borax-buffered formaldehyde-seawater solution.  Samples were then processed in the lab using a dissecting microscope at 10-40X magnification. For samples with high volumes of animals and small detritus, a Folsom splitter was used to subsample. All chaetognaths found were identified to species using Johnson and Allen (2012), and subsequently counted and classified by egg presence. Egg presence was quantified by the visual presence of eggs. If egg presence was visible, then the chaetognath was counted as having eggs present, whereas if no eggs were visible, the chaetognath was classified as having no eggs present.

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