Vertical distribution and composition of phytoplankton under the influence of an upper mixed layer

Abstract

The vertical distribution of phytoplankton is of fundamental importance for the dynamics and structure of aquatic communities. Here, using an advection-reaction-diffusion model, we investigate the distribution and competition of phytoplankton species in a water column, in which inverse resource gradients of light and a nutrient can limit growth of the biomass. This problem poses a challenge for ecologists, as the location of a production layer is not fixed, but rather depends on many internal parameters and environmental factors. In particular, we study the influence of an upper mixed layer (UML) in this system and show that it leads to a variety of dynamic effects: (i) Our model predicts alternative density profiles with a maximum of biomass either within or below the UML, thereby the system may be bistable or the relaxation from an unstable state may require a long-lasting transition. (ii) Reduced mixing in the deep layer can induce oscillations of the biomass; we show that a UML can sustain these oscillations even if the diffusivity is less than the critical mixing for a sinking phytoplankton population. (iii) A UML can strongly modify the outcome of competition between different phytoplankton species, yielding bistability both in the spatial distribution and in the species composition. (iv) A light limited species can obtain a competitive advantage if the diffusivity in the deep layers is reduced below a critical value. This yields a subtle competitive exclusion effect, where the oscillatory states in the deep layers are displaced by steady solutions in the UML. Finally, we present a novel graphical approach for deducing the competition outcome and for the analysis of the role of a UML in aquatic systems.

Influence of oceanographic features on spatial and interannual variability of phytoplankton in the Bransfield Strait, Antarctica

“…This pattern of association (high silicate levels and predominance of cryptophytes) was also emphasized in the late summer of 2010 in the region (Mendes et al, 2013). Relatively high factor loadings were also obtained for the UMLD and water column stability, which together with the high factor loadings displayed by temperature, salinity and Chl-a confirm the strong influence of the water column physical structure on phytoplankton variability as previously noted by several sources (Mitchell and Holm-Hansen, 1991;Ryabov et al, 2010). The ordination diagram showed an unexpected positive relationship between UMLD and stability; however, an expected inverse relation between those two variables has only become apparent after taking the third canonical root into account (see Table 4).…”

Emergence of Holling type III zooplankton functional response: Bringing together field evidence and mathematical modelling

“…I should say, however, that in oligotrophic ecosystems, there is a possibility of alternative density profiles of phytoplankton (i.e. the existence of surface and deep chlorophyll maxima) for the same environmental characteristics (Ryabov et al, 2010). Probably, dynamics of such bi-stable heterogeneous ecosystems can be described adequately only via spatially explicit modelling approach.…”

Tri-trophic Plankton Models Revised: Importance of Space, Food Web Structure and Functional Response Parametrisation

“…In our case, initial supercritical amount of copepods is required to prevent of phytoplankton from overgrazing by microzooplankton or altering the vertical distribution of microzooplankton to enhance the overall intake rate by copepods (such as in Model 1). Interestingly, bistability in vertically distributed plankton ecosystems has been found in some other models [47].…”

“…Even if the overall amount of nutrients is high and the ecosystem is eutrophic, depletion of nutrients in some parts of the euphotic zone, such as near the surface, and their accumulation in other parts (e.g. near the thermocline) can affect the dynamics [17,18,47]. This may require explicit modelling of the nutrient distribution in the water column.…”