Seasonal occurrence patterns of microalgal morphotaxa in tropical seagrass ecosystems of Indonesian coastal bays

This study investigated seasonal occurrence patterns of net-retained microalgal morphotaxa in three tropical seagrass ecosystems at Ekas, Kuta, and Sekotong, Lombok Island, Indonesia. Ten fixed sampling points were surveyed monthly during the rainy season (November 2022–January 2023) and dry season (May–July 2022) within the surface-water layer above seagrass canopies. At each point, 100 L of seawater collected from the surface and the water column above the seagrass canopy was filtered through a 20 μm plankton net, preserved with Lugol's iodine, and examined by light microscopy. Morphological identifications were conducted conservatively, with open nomenclature retained when diagnostic characters were insufficient for definitive species-level assignment. Nine environmental parameters were measured in technical triplicate at each sampling point, and the arithmetic mean of the three readings was used as the monthly value; these measurements, together with bathymetry and regional surface-current fields, were used only to provide ecological context. Following taxonomic reassessment, 52 microalgal morphotaxa were retained, comprising 42 diatom morphotaxa, 6 dinoflagellate morphotaxa, 3 Cyanophyceae morphotaxa, and 1 green microalgal/desmid morphotaxon. For biological analysis, detections across the ten sampling points were consolidated into a single site-month presence/absence record. Seasonal occurrence frequency therefore represented temporal consistency of detection across three monthly observations after spatial pooling, rather than point-level occupancy, abundance, or biomass. The number of detected morphotaxa varied from 23 at Kuta–Dry to 43 at Ekas–Rainy, while 17 morphotaxa were detected in all six site–season combinations. Exploratory hierarchical clustering was used to visualise similarities in occurrence-frequency profiles, whereas environmental variables were presented and evaluated separately. This study establishes a comparative baseline that integrates conservative morphology-based identification, seasonal detection profiles, and environmental and hydro-oceanographic context without inferring causal environmental effects. Its principal limitations are the restriction to the 20 μm net-retained fraction, morphology-based taxonomic resolution, spatial pooling of detections, and three monthly observations per season. Future studies should retain point-level biological records, extend sampling across multiple annual cycles, quantify abundance or biomass, and incorporate molecular or ultrastructural taxonomic confirmation and formal taxa–environment analyses.