Paper 33: Predation, Body Size, and Composition of Plankton

Paper 33: Predation, Body Size, and Composition of Plankton: The effect of a marine plantivore on lake plankton illustrates theory of size, competition, and predation (1965)

By John Langdon Brooks and Stanley I. Dodson

This paper begins by the authors remarking that while the cladoceran Daphnia is present in most of the lakes in southern New England, it is absent from most lakes near the eastern Connecticut coast. They discuss the dominant cladocerans and copepods found in those lakes.

They then discuss the herring-like Alosa pseudoharengus, or alewife fish. This is a marine fish that swims up the streams and rivers that feed into Long Island Sound and ends up in lakes that are within about 40 km of the coastline. They mainly feed on planktonic copepods and Cladocera. The dominant planktons in the lakes with alewife present are smaller sized species, and in lakes without alewife present, the larger Diaptomus spp. and Daphnia spp. are dominant. They speculate that this could be due to predation from the alewife populations.

Changes in Crystal Lake Plankton

They test this hypothesis at a lake in northern Connecticut that had previously been dominated by the larger plankton species prior to alewife introduction in 1942. They sampled Crystal Lake on 30 June 1964, and counted and identified all of the crustacean zooplankton captured both in 1942 and 1964 (Table 1). They also sampled the populations of four lakes with Alosa present, and four lakes without Alosa present. 

They also wanted to know what part body size of the dominants played, and so determined the size range of each species (Figure 4). In 1942, the dominants reached a length up to 1.8 mm. In 1964, no zooplankters over 1 mm could be found. This was smaller in the lakes with Alosa, where the largest species found was less than 0.6 mm in length with an average length of 0.285 mm. In lakes without Alosa, the average length was 0.785 mm. They conclude that Alosa predates heavily on larger species larger than 1 mm in length.

Effects of Predation by Alosa

They now begin to investigate what the significance of this critical size is. They decide that there must be other factors for species between 0.6 - 1 mm in length. Alosa avoid the shores, so predation may fall more heavily on species that avoid the shore also. They discuss the manifestation of this with examples of different species. They bring up the example of one of the Finger Lakes, and state that this one upholds the high numbers of plankton of intermediate size.

Size and Food Selectivity

Here they establish four trophic levels within the lakes to establish the importance of size of food organisms:

Level 4: Piscivores (mainly fish)

Level 3: Planktivores (also mainly fish)

Level 2: Herbivorous zooplankters

Level 1: Microphytes 

They say that animals choose their food on the basis of size, abundance, edibility, and the ease at which it is caught. There is a large difference in food selection between herbivorous zooplankton and higher level predators. Higher level predators tend to consistently choose the largest food morsel available because it requires the least amount of energy to obtain. Visual discrimination plays a large role. In herbivorous zooplankton, on the other hand, visual discrimination has little to no role; their food capturing is determined by the mechanism of removal of the particles from the water in the rate of flow near the mouth. 

Size-efficiency Hypothesis

Planktivores and piscivores are labeled as “food selectors” while herbivorous zooplankton are labeled as “food collectors,” due to their determined food range. 

Size-efficiency hypothesis:

1. Planktonic herbivores compete for fine particulates in the open water.Small particles are most important, composed of algae, detritus and other organic aggregates that provide constant food within the system.
2. Larger zooplankters are more efficient and take larger particles.This is due to greater effectiveness of food collection, reduced metabolic demands per unit mass and greater reproductive success. ( Ex. Daphnia catawba is 4x the size of Bosmina longirostris, so Daphnia will have a filtering area  16x greater than Bosmina.)
3. Therefore when there is low predation, herbivores will be out-competed by larger forms. 
4. When predation is intense, it will take out larger forms, allowing for small forms to be dominant.
5. When predation is moderate, it will keep the larger forms low, allowing for smaller forms to persist.

Prediction tested by Hrbacek et al. and “the result is precisely what the size-efficiency hypothesis predicts.”

Size of coexisting Congeners 

In aquatic and terrestrial systems the common pattern is that larger species take the larger food, while the smaller species eat the smaller food. 

An exception to this rule is congeneric zooplankton living in coexistence are roughly the same size in certain European Lakes. Is this because they are clones??

Summary 

Predation of alewife upon zooplankton, the larger more dominant crustaceans are eliminated and replaced by the smaller species Bosmina longirostris. In regard to planktonic herbivores, the larger species are better at food accumulation due to their more effective strategies. In this case larger species will outcompete the smaller ones when predation is low. However when predation is high, the lager species is removed and the smaller species will become dominant. These demands of competition and predation determine body size of dominant herbivorous zooplankton.