Microphytobenthos Primary Production and Organic Matter Mineralization Along the Sea Level Gradient in a Tropical Mudflat

Tropical tidal flats represent more than half of the global tidal flat area, however information on the ecological and biogeochemical role of microphytobenthos (MPB) in tropical environments remains scarce. The objective of this study was to analyze the changes in sediment biogeochemistry, MPB, community composition and net metabolism along the sea level (SL) gradient in a tropical tidal flat (Punta Morales, Gulf of Nicoya, Costa Rica). Percentage of sediment fine fraction (84.1 ± 12.4%), organic carbon (9.6 ± 1.75 mg cm− 2), and total nitrogen contents (0.8 ± 0.16 mg cm− 2) increased with SL, likely due to higher mangrove inputs and local MPB production, greater deposition of fine particles, and higher preservation capacity associated with reduced oxygen penetration and increased surface area of fine particles. Inorganic nutrients were partitioned among porewater, intracellular and exchangeable pools. Ammonium, phosphate, and silicate were mainly associated with intracellular pools (75, 88 and 62% of total, respectively), whereas nitrate was primarily present in the porewater (68%). MPB abundance (5.9 ± 4.9 × 105 cells cm− 3), dominated by diatoms, decreased seaward, while cyanobacteria were relatively more abundant at higher SL. MPB photosynthesis enhanced sediment oxygenation and likely stimulated aerobic mineralization landward. Net primary production and respiration (2.0 ± 1.51 and 1.5 ± 0.50 mmol O2 m− 2 h− 1, respectively) increased with SL, being the main predictor of ecological and biogeochemical changes across the intertidal gradient. Zonation is essential for accurately scaling up site-specific measurements and for improving estimates of the contribution of intertidal sediments to the overall carbon and nutrient dynamics of coastal ecosystems. Sediment organic matter, organic carbon and total nitrogen increased with sea level. MPB abundance, net primary production and respiration increased with sea level. Diatoms dominate MPB but cyanobacteria proportion increases with sea level. Ecological and biogeochemical zonation are key factors in tropical intertidal flats.