Most people know what algae are, and most have seen both blooms in lakes and pictures taken with a microscope that reveal the fine details of algae cells. Even more people are familiar with fish, even if it is just through the menu at their favorite restaurant. But there are not many fish that eat algae directly; we need an intermediate link to complete the open water food web. That link is the zooplankton, which are small animals, rarely bigger than the head of a pin, that eat mostly algae and are in turn eaten by small fish.
Zooplankton includes small aquatic animals in the water column, mostly crustaceans. The main zooplankters are cladocerans, copepods, and rotifers (see photos), although there are tinier forms (like protozoans) and bigger types (like water mites). They can be filter feeders or selective grazers, picking out what they want from the aquatic soup in which they live. Filter feeders exert the most control over algae, with the filtering capacity proportional to the cube of body length. Consequently, large (>2 mm) bodied cladocerans like Daphnia are preferred for biological control of algae. However, those larger forms also represent the biggest energy “packet” for small fish, and once spring spawning produces the young of the year for each fish species, predation on those larger zooplankton can be intense. Peak zooplankton abundance tends to be in late May or early June, when the quality of algae as food resources is highest, warmer water increases growth rates, and predation by small fish is not yet maximal. This often leads to a clear water phase in even the most nutrient enriched lakes, but it doesn’t last. Types of algae shift toward less edible forms (like many cyanobacteria) and hungry small fish depress the populations of large zooplankton species.
In lakes used by sea-run alewife for spawning, the seasonal pattern tends to be shifted. Spawning alewife don’t eat a lot of zooplankton, but the young of the year live for the summer in those lakes, filtering out zooplankton with their gill rakers and decimating late spring/summer zooplankton. The zooplankton peak in those lakes is usually in winter by evolutionary adjustment. Stocking landlocked alewife in lakes is sometimes encouraged to support trophy gamefish production, but without that long evolutionary adjustment period, this tends to prevent large-bodied, algae-grazing zooplankton from becoming abundant and makes the open water food web much less efficient. It may indeed produce some large gamefish, but at the expense of many other species, possibly the gamefish themselves, as their young need zooplankton for at least a short period in early life.