Bot 351  Paradox of the plankton

Gilmartin & Revelante 1974. J exp mar biol ecol 6:181-204

Tilman 1977 Ecology 58:338-48; 1976 Science 192:463-4

I. Overview

All plankton require essentially the same nutrients
Competition for these nutrients should lead to competitive exclusion of all but the most effective species. Why doesn't this exclusion occur?
Mixed bodies of water typically "house" 30 or more species.
Oceanic to inshore Hawaiian waters house 91 spp of diatoms, 43 spp of dinoflagellates as net plankton
How can this diversity be supported?

II. The Island Mass hypothesis

The Island Mass Hypothesis predicts that the productivity of waters adjacent to islands is proportional to their mass.

Large islands

modify the open ocean by turbulence and mixing in the photic zone
provide limiting nutrients via ground water flux and rain water runoff

Small island do not modify typical tropical structure of open ocean - at least to the same extent as large islands.

    Large islands are relatively rich in species number and productivity (through the trophic structure).

    Gilmartin & Revelante's study was conducted at 4 sites representing a gradient from close to shore to open ocean:
neritic, or coastal area, Kane’ohe Bay
inshore (1 km)
off shore (10 to 20 km)
oceanic (175 km)

Species of phytoplankton were enumerated at each site.

Over 90 diatoms were identified
Multiple patterns for their distribution were realized
Common to all sites
Specific to each site
Overlap in distribution among 2 sites
Overlap in distribution among 3 sites
EVERY possible niche is occupied.

III. Tilman’s tested the mechanisms as bases for ecological competition between two diatoms

Cultures of Asterionella formosa and Cyclotella meneghiniana were grown singly and shown healthy

Michaelis - Menton kinetics can characterize growth rates base on substrate concentration (half saturation kinetic, Km) and ecological success under conditions of nutrient limitation.

Diatom

 Km for Phosphorus  Affinity for Phosphorus Ecological success Km for Silica Affinity for Silica Ecological success

Asterionella formosa

0.04

very high competive dominant w/ Phosphorus limitation 3.9 less competitive excluded w/ Silica limitation

Cyclotella meneghiniana

0.25

 less competitive excluded w/ Phosphorus limitation 1.4 very high competive dominant w/ Silica limitation

 

When cultures were grown together, different nutrient limitations influenced competitive success

Nutrient limitations showed that outcome of growth experiments could be predicted

Setting the ratio of Si/Pi in growth media favors one species over the other

Coexistence is possible if species differ in their ability to acquire and use resources.
At concentrations of limiting substrate where one species is favored by lower Km values, that species should be the competitive dominant.

IV. Outcome:

Co-existance is mediated by both species having physiological limitations - just not the same ones.  Having several resources for which diatoms compete allows for a variety of  species  to co-occur.