A bottom-up perspective on ecosystem change in Mesozoic oceans

Abstract

Mesozoic and Early Cenozoic marine animals across multiple phyla record secular trends in morphology, environmental distribution, and inferred behaviour that are parsimoniously explained in terms of increased selection pressure from durophagous predators. Another systemic change in Mesozoic marine ecosystems, less widely appreciated than the first, may help to explain the observed animal record. Fossils, biomarker molecules, and molecular clocks indicate a major shift in phytoplankton composition, as mixotrophic dinoflagellates, coccolithophorids and, later, diatoms radiated across shelves. Models originally developed to probe the ecology and biogeography of modern phytoplankton enable us to evaluate the ecosystem consequences of these phytoplankton radiations. In particular, our models suggest that the radiation of mixotrophic dinoflagellates and the subsequent diversification of marine diatoms would have accelerated the transfer of primary production upward into larger size classes and higher trophic levels. Thus, phytoplankton evolution provides a mechanism capable of facilitating the observed evolutionary shift in Mesozoic marine animals.

Keywords: ecosystem model; phytoplankton; predation.

Figure 1.

The Mesozoic marine revolution occurred during an extended interval of significant evolutionary change in marine primary producers, including radiations of photosynthetic dinoflagellates (green), coccolithophorids (blue) and, subsequently, diatoms (red). Strontium (Sr) isotopes (grey) suggest a significant enhancement of weathering and nutrient enrichment of the global ocean on the same time scale as (potentially related) diatom diversification. Microfossil diversity replotted from Falkowski et al. [48], based on original tabulations from Spencer-Cervato [49], Bown et al. [50], and Stover et al. [51]; strontium isotope data from Veizer et al. [81].

Figure 2.

Schematic view of the power law relationships between cell volume and key traits of marine phytoplankton. These relationships are rooted in empirical observations and understood in terms of geometric effects on resource acquisition. (a) Maximum growth rate versus cell volume. The solid black line indicates the general trend used in the control model. Mixotrophic dinoflagellates (dotted line) follow the same trend but trade-off a lower growth rate against a generalist resource acquisition strategy. Diatoms (dashed line) are capable of faster maximum growth rates than other phytoplankton. (b) Resource half-saturation for the Monod-kinetics growth model versus cell volume.

Figure 3.

Schematic depiction of the simplified model employed here. A single inorganic resource, R, sustains an assemblage of photoautotrophs (A_i) each of which is consumed by a specific predator (H_i). Cell volume/body size increases with index i. Solid black lines indicate the flow of resource in the purely specialist (autotroph/heterotroph) model. Dashed grey lines indicate the additional flows when mixotrophy is introduced into the model.

Figure 4.

Cumulative biomass (B) with size as a function of resource supply rate (S_R) in the ‘control’ model where maximum growth rate strictly follows the solid black line in figure 2a. The uppermost dashed line indicates total plankton biomass, summing the contributions from each size class of both autotrophs (solid lines) and heterotrophs (dashed lines) which are stacked with contributions from the smallest autotrophs at the bottom, and heterotrophs on top of autotrophs.

Figure 5.

Total system productivity (primary and secondary) as a function of size class in the model at the highest resource supply rate shown in figure 4. Grey bars indicate the control solution where maximum growth rate strictly follows the solid black line in figure 2a. White and black bars indicate the model into which diatoms and mixotrophy were introduced, respectively, as described in the text.