Abstract:
The predictions of the competitive exclusion principle about the number of coexisting species not exceeding the
number of limiting resources in equilibrium constitute an ecological puzzle for phytoplankton ecosystems. Here
we present a synthesizing unit (SU) based competition model taking co-limitation into account, which is the
extension of the competition model developed by Dutta et al. (2014).The study aims at understanding the
mechanisms of violation of competitive exclusion principle for phytoplankton species with seasonal environmental
forcing when multiple resource limitation is taken into account and species growth is formulated based
on SU. We also explore the role of changing environmental conditions on species coexistence on a seasonal and a
decadal time scale by linking the model forcing to the Helgoland Roads Time Series data sets. For the first time,
based on the Helgoland Roads data, we are able to find a realistic parameterization for the phytoplankton
competition model where growth is formulated using SU concept. Our study confirms that more species than
limiting resources can coexist with seasonal variations of environmental conditions. This supersaturation is
related to periodic changes in species’ biomass, variation in interspecific competition and niche configuration,
nonlinear functional response and the position of resource supply within the convex hull of species’ resource
uptake rate. Changes in environmental conditions within realistic ranges do not prevent the coexistence of
species rather it slightly changes species’ biomass and turnover time. This study also confirms that our model
with SU based species growth performs better than species competition model where multiple resource limitation
is formulated based on the product of several Monod functions. Our study has created a new avenue for
phytoplankton coexistence research and the results might be helpful to answer the complex questions on species
diversity maintenance in nature.
