COS 66-4 - Modeling time-variable phytoplankton subsidy reveals at-risk species in a Chilean intertidal ecosystem

Abstract: The intertidal ecosystem off the coast of Chile requires supplemental primary productivity to sustain its rich biodiversity. This productivity is largely derived by wind and wave action transporting phytoplankton from the ocean toward the coast. Previous work by our group and collaborators have modeled population dynamics in this ecosystem using the allometric trophic network (ATN) modeling framework. This previous work modeled oceanic phytoplankton subsidy as a constant influx of biomass to the intertidal phytoplankton node. This work provides a thorough understanding of the network’s asymptotic response to high, low, and zero phytoplankton subsidy.

Here we propose integrating time-variable, empirical, chlorophyll-a measurements into our model as a proxy for offshore phytoplankton biomass. We use this variable offshore phytoplankton to drive intertidal population dynamics in real time. We hypothesize that there are significant transient dynamics introduced by the stochastic nature of offshore phytoplankton abundance, and that these transient dynamics are lost when examining the asymptotic models. Further, we hypothesize that incorporation of empirical measurements will improve modeling realism and reveal novel insights into population dynamics that otherwise are lost when examining asymptotic results.

We obtained daily measurements of chlorophyll-a concentration from 1999-2010 from our collaborators at Estación Costera de Investigaciones Marinas near Las Cruces, Chile. This dataset was cleaned, processed, and connected to the existing Chilean intertidal network in a manner which introduces a new rate constant representing oceanic-to-coastal mixing. Our strategy reveals bottom-up control over many ecosystem aspects including: (1) differential responses to phytoplankton abundance between closely related sea snails, arising as a result of trophic network structure; (2) the direct relationship between the rate of oceanic-to-coastal mixing with the magnitude of biomass fluctuations, with higher trophic level species like crabs and gulls experiencing magnified fluctuations; and (3) the inverse relationship between the rate of oceanic-to-coastal mixing and species persistence, with a specific sea slug Onchidella spp. being identified as particularly vulnerable to high fluctuations in phytoplankton subsidy.

Understanding the effects of nonautonomous dynamics on ecological systems – such as those imposed by stochastic environmental drivers -- is an important new frontier in community ecology. Our work extends the ATN methodology by creating a straightforward framework for the incorporation of such variable, external driving forces.