Seagrass responses to light on different timescales: how do models fit in?

Light is a critical requirement for seagrass survival, and light deprivation is a major contributing factor to the loss of seagrass meadows at a global scale. However, light availability varies on multiple timescales, ranging... [ view full abstract ]

Light is a critical requirement for seagrass survival, and light deprivation is a major contributing factor to the loss of seagrass meadows at a global scale. However, light availability varies on multiple timescales, ranging from seconds to years. The responses of seagrasses to this variable light environment are complex, and analyses using modelling approaches can help to identify the dynamics that likely underlie the ecological processes. This talk presents our recent work combining field data and modelling approaches to identify (1) how seagrass ecosystem metabolism responds to light history over the diel cycle and (2) how seagrass biomass responds to light history on seasonal timescales.

First, we analysed the diel pattern of O2 flux between a seagrass ecosystem and the water column for three depths within a monospecific Zostera muelleri meadow in Swansea Shoals, NSW. Three separate models, each representing different ecological mechanisms, were fit to the data to identify the mechanism most likely to explain the distinct pattern observed in O2 flux, which was a model of diel variation in ecosystem respiration driven by light history. Ecological mechanisms which are potentially responsible for this best model fit are discussed.

Second, we analysed field data sets for daily light and seasonal biomass of several seagrass meadows from the Great Barrier Reef and two lakes in NSW. The correlation of different light history indicators to biomass, for different periods of light history, was compared. The indicators tested ranged from an unweighted mean light history period (typically used to inform statistical models) to a rolling average of light history which analytically approximates an ordinary differential equation of growth/mortality kinetics of seagrass biomass (typically used in deterministic models). The relative merits of each indicator, and the recommended light history period that should be considered in seagrass monitoring programs as a result of this work, are discussed.

Mathematically, the dynamics of light history considered in these studies is highly analogous to previous models of photoacclimation responses in corals and phytoplankton. To conclude, I demonstrate how the mathematical concepts used here can be applied to investigate photoacclimation and/or light history kinetics in any aquatic ecosystem.