The Indian River Lagoon estuary has been suggested to be in transition from a benthic productivity state to a phytoplankton-based state because of nutrient inputs, phytoplankton blooms, and light penetration. Understanding features such as phytoplankton blooms and subsequent effects on light penetration, dissolved oxygen, and benthic systems is challenging because of the underlying complexity and analyzing direct effects can sometimes produce misleading results.
Many factors operating alone and in concert affect biotic communities. The suite of physio-chemical, biotic, and other factors establish a complex ecological topology. The overarching hypothesis (or the ‘one SEM to rule them all’ hypothesis) states that, for the entire IRL and all 21 years, nutrients, temperature, distance to inlet, and other factors have direct and indirect effects on phytoplankton biomass, which can then influence ecological functions such as changing the dissolved oxygen and light penetration regimes of the water column. This hypothesis, and other hypotheses involving dividing the IRL into regions and multi-year temporal segments for analysis were subjected to Structural Equation Modeling.
Results/Conclusions
A structural equation model including linkages among chlorophyll, nutrients, salinity, light penetration, dissolved oxygen (D.O.), etc. explained phytoplankton biomass (Chl-a, R2=0.46) and relative abundance (Chl-b and c) and ecological functions (light penetration R2=0.60, and D.O. R2=0.42) across the IRL and over the >20 years of monitoring data. Results indicate that IRL experiences nitrogen and phosphorus co-limitation of phytoplankton, and that changes in plankton biomass influence ecological functions of D.O. and light penetration. Partitioning the IRL into regions improved the model fit to observations and showed that different regions of the IRL had different drivers for phytoplankton biomass. Comparing three-year groups of data for long and short water residence time regions showed that phytoplankton blooms in the 2010-2012 period (including the so-named “superbloom”) were driven primarily by nitrogen and less by phosphorus and that this was associated in part by decomposing phytoplankton producing high concentrations of phosphorus.