Abstract
Coral atoll lagoon sand cays and islands are common landforms in the Maldives. Their biogeomorphic development has received much less attention than the larger rim islands, which enclose the lagoons. Lagoon sand cays appear to be more dynamic and may form over comparatively short time scales (< 10 years). They accrete on subtidal patch reefs from biogenic sediment derived from the surrounding reef platform; and are evolving and being colonised by marine-dispersed plant species over periods of months to years. Plants may play a critical role in island emergence, development and stability. This thesis will investigate where and how early colonising plants establish on lagoon sand cays and environmental constraints on plant growth, including nutrient conditions, salinity, a freshwater lens and shoreline morphodynamics.
The biogeomorphology of two lagoon islands in Huvadhoo Atoll, Republic of the Maldives, is examined. Maahutagalaa is an established, forested, island, whereas Maaodagalaa is a dynamic sand cay. Both experience ongoing colonisation by plants, erosion and progradation. Local sea level is derived by relating RBR deployments to topographical surveys; while regional sea level is estimated by superimposing a regional datum (Gan tide gauge data) against RBR deployments, which are similar in magnitude and time. Early successional species established across a terrace on Maahutagalaa in strand lines between 2014 and 2017 (elevation of terrace is 0.40–0.70 m above local datum, spring high tide on the 20/04/2019). Plants established on Maaodagalaa II (June 2019, maximum elevation 0.62 m above local datum, SHT 12/02/2020) after the sand cay migrated 100 m northwest between June 2019 and February 2020, transitioning from Maaodagalaa I (February 2020, maximum elevation 1.03 m above SHT 12/02/2020).
Changes in morphology and vegetation cover are identified and recorded on these islands during fieldwork in June 2019 and February 2020 using consistent methods over periods of years (satellite imagery); to months (UAV photogrammetry; vegetation surveys; laser level profiles). Estimates of wave run-up (empirical equations; H1/3 0.30–0.98 m; T1/3 2.29–4.13 s) and astronomical tides (recorded during RBR deployments and estimated from a long-term tide gauge record at Gan between 1987 and June 2020) are related to topographical surveys of the islands. The absolute and relative importance of environmental constraints (erosion; salinity; availability of freshwater; and nutrient availability in soils) to plant growth are estimated. This included monitoring inundation events (time-lapse camera); erosion and progradation (satellite imagery and UAV photogrammetry); and sampling the groundwater (RBR, SolinstTM water level recorder) and soil (at the surface, then at depths of 10 cm, 20 cm and 40 cm).
This study has identified two environments where early colonising species can establish on lagoon islands. Scaevola taccada, Suriana maritima, Lepturus repens and Cyperus conglomeratus, and a few other species, have established across Maaodagalaa II and the northern terrace of Maahutagalaa. This study has identified and quantified marine conditions that are needed to achieve a successful stranding event. A wave event, resulting from a combination of distant source swell and/or wind-forced waves, is required to overtop the northern scarp on Maaodagalaa II (0.32–0.45 m above local datum, SHT 12/02/2020) and the seaward ridge (northern beach) on Maahutagalaa (0.64 m above local datum, SHT 20/04/2019), where past inundation did not occur due to tsunami. Wave run-up was recorded and measured on the 12th February 2020 on Maaodagalaa II, where the northern scarp was overtopped and inundation reached up to 0.50 m above local datum, SHT on 12/02/2020 and 15 m inland of the scarp. High (mean 8.1 m s-1; 4.6 m) onshore (55o) winds were recorded for 24 hours prior to inundation, and are predicted to form wind-forced waves (H1/3 0.50–0.80 m, T1/3 2.8–3.5 s). These waves are predicted to inundate the northern beach up to 0.50 m above the still water level (SHT 12/02/2020), where maximum water depth at the beach toe (16 m from the northern scarp) was 1.41 m.
Vegetation cover expanded to 616 m2 on Maaodagalaa I and 6,080 m2 (terrace 1299 m2; forest 4,781 m2) on Maahutagalaa between 2014 and 2019. Vegetation cover was largely destroyed on Maaodagalaa I (520 m2) and the forest margins (175 m2) and northern terrace (211 m2) on Maahutagalaa, during island shoreline erosion between June 2019 and February 2020. Plant growth has not been constrained by the low nutrient conditions in the soils of Maaodagalaa I and Maahutagalaa (northern terrace) (NH4-N and NO3-N median < 0.5 ppm; organic carbon 12.5–13.6 g kg-1); while soil NO3-N and NH4-N (1–6.5 ppm) and organic carbon (13.1–20.4 g kg-1) is significantly (P-value < 0.05) higher in soils from the forest on Maahutagalaa, possibly due to higher plant biomass; nutrient inputs from animals; and age of soil (forest first visible in 1969 aerial photograph). Inundation occurred on Maaodagalaa II on 8 days between the 13th of February and 26th of July 2020, exposing vegetation (present up to 0.62 m above local datum, SHT (12/02/2020)) to salt water. Inundation did not constrain plant growth, based on time-lapse images. Inundation occurred in combination with perigean SHT (1), SHT (1), and HT either side of SHT (6), elevation 2.08–2.36 m (regional datum). The availability of freshwater and salinity of groundwater, measured on Maaodagalaa II (0.75–0.90 ppt, 40 cm thick), did not constrain plant growth on Maaodagalaa or Maahutagalaa. Extended periods of salt stress and/or droughts may constrain plant growth, based on leaf necrosis occasionally visible on plants.
A model of plant colonisation of lagoon islands is proposed. This model involves the interaction of astronomical tides and wind-forced waves, which may, in combination, inundate an island surface and strand seeds of early colonising plants (and wrack and sediment) above the usual elevation of SHTs. The coincidence of waves and SHTs is crucial if seeds are to strand at an elevation where they may germinate and grow, without exposure to inundation by waves, during lower tides over the perigean SHT cycle. Once an island starts to accrete, inundation may constrain plant growth, by exposing plants to salt water. Plant growth may be partially destroyed during episodes of island erosion, but is not constrained by the availability of freshwater or nutrients (low) in soils. The successful stranding, germination and establishment of early colonisers may be an important step in island emergence, development and stability.