Taxonomic Composition and Richness Equilibria in Phytoplankton Communities

The equilibrium concept in phytoplankton communities refers to the steady state of a set of phytoplankton populations which are coexisting and hence 'sampled together'. This means that while growth and loss processes occur simultaneously in the populations of a guild, it is possible that the result of these processes is the persistence of composition over time.

Persistence can be measured on the timescale of significant environmental variability as well as the turnover and generation times of individual species. In phytoplankton communities, where generation times are measured in hours and days, a few weeks of similar densities and community composition can be considered indicative of equilibrium or steady-state persistence. Cases of steady-state persistence are: the co-dominance of five species (Planktothrix agardhii, Limnothrix redekei, Dictytosphaerium sp., Cyclotella meneghiniana, and Cryptomonas erosa) over nine unperturbed weeks in El Porcal Lake (a gravel pit in Central Spain); the dominance of a number of species in 31 fluctuating sites of a wetland (La Safor, Mediterranean Spanish coast); and the persistence of some nondominant species (Peridinium willei and Planktonema lauterbornii) over more than 3 weeks in the water column mixing period in Las Madres Lake (Central Spain).

The observation of persistence within phytoplankton communities is also related to the spatial and temporal scale of the observer. From a practical point of view, it is a rule for researchers to sample at least weekly. This is because the time generation of microalgae is from 0.3 to 3 days, so a week is a representative timescale of population response. From a statistical point of view, a similar assemblage must be found in at least three successive samplings in order for it to be considered stable. Therefore, the minimum timescale commonly utilized to evaluate persistency may cover more than 50 generations of the smallest species.

The equilibrium concept in phytoplankton communities is supported by evidence of nutrient and resource limitation, of inter-specific competitive coexistence and of trait-specific competitiveness ability among phyto-plankton species. The high resilience of phytoplankton communities also supports the achievement of equilibrium conditions among species pairs or species groups on a given time and spatial scale. The relatively small number of dominant species in phytoplankton communities supports this simplification.

However, the equilibrium concept in phytoplankton communities is not supported by the large number of coexisting species in a medium which can be considered relatively isotropic and unstructured; nor is it supported by the well-known tradeoff between population growth rate and stability. These considerations, which were first raised by Hutchinson, led to the famous 'paradox of the plankton' and disequilibrium or nonequilibrium theories.

Therefore, the two branches of theories (i.e., equilibrium vs. nonequilibrium) address two different aspects of phytoplankton community structure: the relationship between pairs or small groups of species on the one hand, and the organization of phytoplankton communities as a whole, on the other hand.

Equilibrium Theories

The original description of persistent or steady-state phytoplankton communities was focused on dynamic features of temporal changes in phytoplankton and was based on resource-partitioning concepts, and more specifically on competition. The Tilman extension of the Volterra equilibrium model to primary producers represented the basis of equilibrium theory in phytoplankton communities.

To explain the 'plankton paradox', equilibrium theories propose that several species ofphytoplankton can coexist in true competitive equilibrium if they are collectively restricted from further growth by different nutrients. The relative abundance of the coexisting species can thus be controlled by the ratio of limiting resources. The hypothesis assumes that (1) several nutrients are in relatively short supply; (2) growth of each species is restricted by a single nutrient or a unique combination of several nutrients, according to the Liebig's law principle that the growth of each population occurs at the rate permitted by the most limiting factor; and (3) different species have different uptake capacities for the various nutrients. Equilibrium approaches to the phytoplankton paradox are deduced theoretically and verified in chemostat experiments. Equilibrium conditions between species pairs on two limiting resources can be described in terms of the competitive ability of each species with respect to each resource, as described by the resource requirement at equilibrium:

where R* is the equilibrium extracellular resource density, ri is the maximum growth rate, Ki is the resource concentration at which growth is half of the maximum growth rate and D is the dilution rate, taking account of resource concentrations at the start and resource inputs; the result of the equilibrium can be also derived graphically (Figure 3).

Laboratory competition experiments have supported the relevance of resource competition as a process that determines the species composition of phytoplankton communities. Under chemostat-type steady-state conditions phytoplankton species limited by different resources can coexist in equilibrium; and the relative abundance of coexisting species is controlled by the ratio of limiting resources.

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Figure 3 Potential coexistence of two species for two limiting resources. ZNGIi (i = A and B) curve represents the amounts of the two resources which must be available for the species i to maintain an equilibrium population. Dashed lines are consumption vectors (CA and CB) of the species A and B, which represent the total consumption rates of resources by the species at equilibrium. ZNGI's curves cross at a two-species equilibrium point. ZNGI's curves and consumption vectors define six regions. Habitats which have resources points within the region 4 will have both species coexisting, while species A will dominate in habitat 2 and 3, and species B will dominate in habitat 5 and 6. Reproduced from Tilman, David; Resource Competition and Community Structure. © 1982 Princeton University Press. Reprinted by permission of Princeton University Press.

Tilman has shown this with 76 competition experiments between the two species of freshwater algae Asterionellaformosa and Cyclolella meneghiana under a wide range of Si:P ratios.

As results of these experiments Asterionella formosa was observed to be competitively dominant when both species were phosphate limited; Cyclolella meneghiana was dominant when both species were silicate limited; and both species stably coexisted when each species was growth rate limited by a different resource. Extension of this approach from a two-species system to multispecies experiments has supported the equilibrium theories; however, they are able to explain only a small proportion of the diversity and species richness of natural phytoplankton.

The Plankton Paradox, or 'Supersaturated' Communities

The remarkable diversity ofphytoplankton communities in aquatic ecosystems is recognized as a 'paradox', because many more species with similar requirements (to be satisfied from the surrounding environment, apparently isotropic or unstructured) co-occur than is expected in a competitive equilibrium. The high level of richness of phy-toplankton communities in the same water body has been called by G. E. Hutchinson the 'plankton paradox' and the resulting communities are defined as 'supersaturated'.

The plankton paradox was based on the assumption that: (1) phytoplankton guilds are assembled on the basis of differential growth rates among species, determined by the availability of inorganic nutrients; (2) species interact through competition for mineral nutrients, and (3) pelagic habitats are closed homogeneous systems. However, pelagic habitats are open systems, intraspecific competition is generally stronger than interspecific competition, and nutrients (as well as some other limiting factors) vary in time and space at different scales.

Therefore, in general terms, competitive equilibrium is never expected when a virtually complete competitive replacement ofone species by another occurs in a time (tc) of the same order as the time (te) taken for a significant seasonal change in the environment which reverses the competitive ability of the interacting species. Thus, ideally there are classes of cases:

1. tc << te, competitive exclusion at equilibrium complete before the environment changes significantly;

2. tc ffi te, no equilibrium achieved; and

3. tc >> te, competitive exclusion occurring in a changing environment to the full range of which individual competitors would have to be adapted to live alone.

Case 2 gives rise to a number of nonequilibrium theories concerning phytoplankton communities. A nonequili-brium model assumes that fluctuations or disturbances occur with sufficient frequency to disrupt the course of competitive exclusion, but not so frequently as to force species to adapt to the overall variability. High biodiversity is attained when fluctuations or disturbances keep the competing populations far from equilibrium, thus allowing more species to coexist.

Two types of fluctuation can be distinguished.

1. Fluctuations caused by internal nonequilibrium dynamics generated by competitive interactions.

2. Fluctuations caused by external factors, such as oscillation in light and nutrient supply due to the seasonal cycle or less predictable factors such as changes in weather and hydraulic conditions. Moreover, spatial heterogeneity is an important reason to expect coexistence of species. Even in seemingly homogeneous environments, such as the open ocean, meso-scale vortices and fronts generate barriers preventing complete mixing and competitive exclusion.

Nonequilibrium Theories

Fluctuations determined by internal feedback factors

It is usually thought that in the absence of any externally imposed environmental fluctuation, phytoplankton communities will approach competitive exclusion

and ecological equilibrium. However, phytoplankton continue to fluctuate erratically even when the environment is completely constant and uniform. This is because the high turnover rates and feedbacks in the phytoplank-ton system itself generate complex dynamics preventing the system from coming to equilibrium, the resulting community being referred to as 'supersaturated'.

It has been shown that competition for limiting resources leads to chaotic dynamics if multiple species compete for at least three resources. Theoretical models have shown that these dynamics depend crucially on the relationship between the resource requirements and the resource consumption characteristics of the species. Competition generates: (1) stable coexistence, if species consume most of the resources for which they have high requirements; (2) oscillations and chaos, if species consume most of the resources for which they have intermediate requirements; and (3) competitive exclusion, with a winner that depends on the initial conditions, if species consume most of the resources for which they have low requirements.

Fluctuations determined by external factors

Environmental disturbances that occur so frequently as to preclude competitive exclusion in phytoplankton species lead to a nonequilibrium, promoting coexistence and enhancing diversity. General analytic models show that resource supply in a situation ofnonequilibrium can allow coexistence of species competing for the same fluctuating limiting resources. Theoretical results showing that resource fluctuations allow coexistence have been confirmed experimentally for phytoplankton competing for key nutrients (phosphorous and ammonia) and light. The coexistence-promoting mechanism believed to operate in variable environments is the creation of temporal niche opportunities that allows competitors to utilize the scarce resources at different times.

The coexistence of two or more species under variable resource supply conditions is possible when species exhibit a gleaner-opportunist tradeoff that entails a tradeoff between a low minimum resource requirement and a high maximum growth rate. A gleaner species grows better at low resource levels as a result of a low minimum resource requirement. An opportunistic species is one that can take advantage of high resource levels.

Environmental fluctuations such as a pulsed nutrient supply can increase phytoplankton species diversity, facilitating coexistence. In equilibrium theory, the gleaner or 'affinity specialist' is able to exclude the other competitors if the system tends toward the equilibrium resource level of the minimum R* value. Fluctuations in resource levels can prevent this. If nutrients are sinusoidally fluctuating, then a species with a high growth rate, able to take higher advantage of high resources levels (i.e., opportunistic or velocity specialist), might coexist with a slower-growing but more efficient (gleaner) population. These results can be generalized to a wide class of growth functions and to pulsed variability of nutrient supply simulating periodic upwelling events.

Light fluctuations may also have an effect on the outcome of competition between phytoplankton species, leading to nonequilibrium coexistence. Light is never homogeneously distributed, even in a well-mixed water column; the light intensity at the surface always exceeds the light intensities at the bottom and a spatial gradient in light distribution with depth, coupled with diffusion of algal cells through the water column, may allow coexistence of many species of phytoplankton as well as their vertical segregation. Temporal variability in light supply can also have significant effects on the outcome of competition between phytoplankton species when species exhibit the gleaner-opportunist tradeoff.Rapid light fluctuations can reverse competitive dominance from a gleaner to an opportunist; slow light fluctuations can change the identity of the dominant competitor and also lead to the stable coexistence of competitors. Coexistence is easiest between species that are highly differentiated along the gleaner-opportunistic tradeoff.

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