MS DEFENSE | Lani Musselman | Investigating The Drivers Of Phytoplankton Community And Productivity Dynamics In An Indigenous Managed Aquaculture System
Title: Investigating The Drivers Of Phytoplankton Community And Productivity Dynamics In An Indigenous Managed Aquaculture System
Phytoplankton form the foundation of marine ecosystems, yet quantifying primary productivity in shallow coastal environments remains challenging because satellite-based estimates are unreliable in optically complex waters, making productivity assessments in these systems dependent on direct field measurements. This study investigated nutrient drivers of phytoplankton community dynamics in He‘eia Fishpond, an ~800-year-old Hawaiian loko i‘a (Native Hawaiian aquaculture system) undergoing biocultural restoration. Four nutrient amendment experiments were conducted across wet and dry seasons between July 2023 and March 2024) to examine nitrogen and silicate enrichment effects on phytoplankton productivity, biomass, and community composition. Measured parameters included productivity via 13C, phytoplankton size composition via size-fractionated chlorophyll-a, for microbial cell abundances via flow cytometry, and eukaryotic plankton diversity and community composition via 18S rRNA gene sequencing.
Results demonstrate that inorganic nitrogen availability likely drives phytoplankton productivity, with nitrate additions consistently stimulating growth and biomass accumulation. Maximum net primary productivity reached 500-600 mg C m-3 d-1 under nitrogen enrichment, representing up to 20-fold increases over rates in control conditions. Ambient nutrient stoichiometry consistently deviated from Redfield ratios (N:P < 16:1) at this sampling location. Community responses exhibited nonlinear dynamics dependent on baseline functional diversity: high-diversity assemblages maintained stability through temporal niche partitioning, while synchronized blooms of ecologically similar taxa drove competitive exclusion despite moderate enrichment. Pico- and nanoplankton (0.7–20 μm in size) dominated productivity responses, highlighting the disproportionate ecological role of smaller size classes in this ecosystem. Silicate additions alone produced minimal enhancement and favored mixotrophic taxa capable of circumventing nitrogen limitation.
These findings provide the first direct productivity measurements at Heʻeia Fishpond and suggest that inorganic nitrogen may be a key driver of phytoplankton dynamics. While spatial and temporal scope limits pond-wide conclusions, this work provides a preliminary ecological rationale for management interventions enhancing inorganic nitrogen delivery to support the growth of Hawaiian striped mullet (‘ama‘ama, Mugil cephalus) and establishes a methodological framework for future loko i‘a research.
Location: Moore Conference Center (C-MORE)
Date & Time: Thursday, February 5th, 2026 at 1:00pm or Zoom (Passcode: piko)
