Coastal phytoplankton community dynamics, interactions, and coexistenceSubmitted by editor on 30 October 2018.Get the paper!
Diatoms and more generally phytoplankton exhibit a striking diversity of species, which is reflected in a stunning variation of shapes, sizes, and life-histories.
Photo credit: Nadine Neaud-Masson - Ifremer Nantes
Yet, being primary producers, these small algae ought to compete for the same basic nutrients in a media that is often continuously stirred. This begs the question about how they coexist, the so-called “paradox of the plankton”, as Hutchinson put it in 1961. Hutchinson proposed an explanation: temporal fluctuations in the environment could counteract competitive exclusion. Theory has later shown that it is mostly true but a little subtler than that - only mechanisms which make the populations bounce back from low densities (e.g, the storage effect where environmental variation covaries with competitive strength) are able to allow coexistence for extended periods of time without requiring additional immigration.
Can coexistence occur through temporal variation like Hutchinson imagined? Or do nutrient niches explain community dynamics and diversity maintenance? These theoretical questions can be tested using dynamical models, but data is often lacking.
During my first visit to Bordeaux in 2015, I met our colleague Isabelle Auby from Ifremer (the French institute doing most of the monitoring of marine ecosystems) who mentioned that Ifremer was collecting long-term data on phytoplankton in the nearby Arcachon Bay. For somebody used to time series of small mammals, often 50 or 100 time points at best, community-level data every two weeks and going back to the 80’s seemed like a goldmine. This allowed to put ideas about coexistence and drivers of community dynamics to the test. Our group at U Bordeaux teamed up with Ifremer, and fitted autoregressive models to uncover biotic and abiotic forces shaping phytoplankton dynamics.
Our initial interest was to link phytoplankton community dynamics to nutrient inputs from the land. As we discovered later though, nutrients in that area seem to be in that Goldilocks zone, where there is neither too much nor too little nutrient for them to have a massive impact on phytoplankton long-term dynamics. Other abiotic factors like hydrodynamics and light (of course) seem to have more impact on community dynamics.
Chaetoceros decipiens - a common player in our community. Photo credit: Nadine Neaud-Masson, Ifremer Nantes.
We did, however, find neat results about coexistence. There has been a broad debate in ecology about whether neutral theory vs. the niche allows to explain coexistence. For some plant species, the evidence seems currently to sway towards niche theory. In phytoplankton, the jury is still out. We show in our paper that intraspecific competition (intra-genus, actually) is much stronger than interspecific, which is impossible under neutral theory (where everybody competes by an equal amount irrespective of species identity). In fact, among the weak interspecific interactions that we found, many are positive. These elements tend to points towards natural enemies (predators like zooplankton, parasites like viruses or perhaps fungi) as proximal causes of coexistence. Spatial structure at the microscale may also play a role in decreasing competition.
Thus coexistence occurs through stabilizing mechanisms in this coastal community, but apparently not through nutrients. And we do not need to involve the storage effect or relative nonlinearity mechanisms often invoked to resolve the paradox of the plankton.
While we have shed some light on how phytoplankton coexist, there is still much to discover. First, we studied coastal phytoplankton. Nothing guarantees that the same mechanisms would hold up in the open sea where nutrient limitation might be more frequent. Second, we used data obtained using an optical microscope. This means we study the relatively large phytoplankton, at the micrometer scale (although it’s called microphytoplankton, there are the smaller nano- and pico-phytoplankton). Small species make up a large pool of the marine biomass and their regulating factors could differ.
As a final note, this study shows how much long-term data can bring to both applied ecology (it was originally designed to detect harmful algal blooms) and fundamental ecology. It is crucial that we preserve such long-term data collection observatories.
The authors through Frederic Barraquand