Report on seagrass and Halimeda monitoring in Lac Bay 2020

Historical Data and Background

The first known qualitative seagrass survey in Lac Bay was conducted in 1969. Sincethen, additional quantitative surveys were conducted in 1999 and 2007 and on a regularbasis since 2011 (2011, 2013, 2015, 2017, 2018,2019, 2020).After each survey, raw dataforseagrass observationswereuploaded to the DutchCaribbeanBiodiversityDatabase (DCBD)and have beenreferenced innumerouspublications, most notably the seagrassandsea turtle research byChristianen andSmulders from Wageningen University and Research (WUR).In 2010, the invasive speciesHalophila stipulaceawas first documented in Lac Bay, andhas continued to spread since. This is a concern as this species is fast growing andhas been known to outcompete local species of seagrasses. If left unchecked,Halophila stipulaceais a threat to native seagrasses and the species of which dependon it (such as green turtle and queen conch).

Discussion

In recent monitoring (2019, 2020) pictures were taken of all quadrants surveyed as wellas of the surroundingarea.SeagrassOverall, there has been a decrease in the native species ofT. testudinumand anincrease in the invasive speciesH. stipulacea. S. filliformepopulations appeartobe stable, with a slight increase in coverage.Sargassum has been an issue within Lac Bay, with several of the survey sites beinglocations where decaying sargassum has created a thick mat, which in most cases wasslowly removed with the tide. Two ofthese sites, Locations A and G were completelycovered by sargassum. Physical repercussions of the sargassum landings can be seenby the seagrass dieback all along the mangrove border from the south until just north ofPunto Kalba.Additionally, at location E, a very fluffy sediment was found to be covering the substrate.A likely explanation is that this is the result of decomposed sargassum settling at thissite.The overall cover by all species together seems to be stable, but in terms of biomass itwould appear to be lower. The ecosystem services provided byHalophila stipulaceaaresignificantly lower than those ofThalassia testudinumdue to its shallow root structure(Smulders et al., 2017) and the fact that it is less nutrient rich than native seagrassspecies (Boman et al., 2019).The shift towards this nonnative species is of concern andshould be closely monitored.

Halimeda

Since 2018 Halimeda species and bioturbation observations were added to themethodology of these surveys. 2020Data onHalimeda sp.occurrence can be found inAppendix III.Overall cover by Halimeda seems to have decreasedbut a longer time series isrequired to draw more definitive conclusions. Two students have looked into carbonatesand production by Halimeda during theLac Ecological Restoration project: LauraTimmermans (2018)and Valeria Pesch (2019). Results from these studies wereinconclusive, highlighting the need for additional research to fully understand thecontribution of Halimeda to carbonate sands and infilling of the bay. In addition, moreinformation is needed on the influence of eutrophication (Slijkerman et al., 2011) on thisprocess.

Sand particles size in Lac was measured during the Conch Stock Restoration project.Larger fractions often show Halimeda segments next to small shells and othercarbonate particles (Figure 5). For this reason, it is believed thatHalimeda sp.are amajorcontributor of sand within the bay.Sediments have been analyzed for carbonate content in several other studies such asthe EHLZK project and during the baseline surveys conducted in 2012 (Debrot et al,2012). Although the findings have not been published, the data showed that sedimentstowards the center of the bay have a higher CaCO3content, and the distribution sand,silt, clay changes (Appendix V). In addition, it was found that terrigenous sedimentswere most prevalent along the borders of Lac-mainly in the northwestern sector,whereas endogenous sediments were found in the central part of the bay and towardsthe reef.

Bioturbators

Bioturbators have also been added to recent surveys as it is believed to be important asit may cause a loss of sequestered carbon, and new sediment may facilitate settlementofH. stipulacea. Bioturbators are considered to be ecosystem engineers, changingthesubstrate landscape. Common bioturbators are callianassid (burrowing ghost) shrimp,the lugworm, mantis shrimp and the burrowing sea cucumber. To date, surveys haveonly included rough estimates as to the number of mounds per square meter.After discussing this issue among local researchers, it has been decided to amend theprotocol for next year to count all mounds in the same quadrant as the seagrass cover.If more than half of the mound falls out of the quadrant the mound will not be counted