Aquatic Food Web and Aquaculture

The past week I've spent a lot of time with my new toy, the microscope and realizing the difficulty of properly identifying phytoplankton/zooplankton, decided to order a few select books which may can help in that arena.
One of those books I chose after an extended surf on amazon is Pennak's Freshwater Invertebrates of the United States. I received the book just yesterday, have skimmed over it and rather pleased with the sheer number of illustrations, though the vast majority are line-drawings and not in color. Nonetheless it appears it will make a valuable reference book on freshwater invertebrates. There's a large section dedicated solely to rotifers or, properly rotifera (Wheel animals) Chapter 7 pgs. 129-190, a subject that's raised often in Aquaculture. It contains a rich collection of illustrations to help identify individual species of rotifers.
From looking it over, it seems this book would be best approached if you read the introduction first instead of skipping to latter chapters. The introduction seems to be plentiful in information that brings the reader up to date on issues related to freshwater invertebrates.

In our class, the instructor gave me a copy of Zooplankton Succession and Larval Fish Culture in Freshwater Ponds, by Gerald M. Ludwig, (a full list of documents in pdf format, are listed here http://srac.tamu.edu/fulllist.cfm). In this paper it tells the proper timing to add fish to the pond. You don't want to put the fish in too early, for lack of plankton (their food source), and too late can result in the zooplankton eating the fry:

Tiny fry eat only tiny prey, but tiny fry are preyed upon by many creatures bigger than they are. It is important to know the size of the fry you are stocking and to make sure that the pond you are putting them into contains plankton of the size that will be their prey and is also void of creatures that will prey on the fry.

Fish fry eat zooplankton, phytoplankton and tiny plants and animals attached to objects on the pond bottom... Most fish fry eat three main types of zooplankton-- rotifers, copepods and cladocerans. For the tiniest fish fry, small rotifers may be the only zooplankton small enough to eat. For larger fry, the smallest rotifers may not provide enough nutrients to make chasing and ingesting them worth the effort. Copepod nauplii, which are just hatched copepods, are important first foods for larval fish, too. Protozoans may also be eaten, but little is known about their contribution to fry diets.
- Zooplankton Succession and Larval Fish Culture

The document goes on to tell the stages of succession in ponds, the first group appearing in ponds are the rotifers which obtained their name from their "wheel organ," a ring of cilia that appears to rotate around the mouth. Rotifers can reach maturity in 2-8 days, and hatch from "resting eggs" that survived on the bottom of the pond while it was dry. Most are asexual, until conditions are harsh, then they become sexual and and resting eggs are produced again.
Rotifers are followed by copepod nauplii, the second largest group to appear in ponds, and reach maturity about 18 days after the pond is filled. With their growth (up to 2-3mm) they provide larger food source for larval fish. Only larger fry such as catfish have large enough mouths to eat adult copepods.

Although copepods may be prey for larger fish fry, sometimes the roles are reversed. Introducing small fish fry into a pond full of large copepods can be disastrous... When cyclopoid copepods are prevalent, they may eat all the fish fry stocked.
- Zooplankton Succession and Larval Fish Culture

The document explains proper timing of fry stocking, such as "the proper timing of fry stocking in relation to filling and fertilizing the ponds, can make the difference between having an abundant harvest or a complete crop loss... and optimum growth of fry."
The entire document can be downloaded at http://srac.tamu.edu/fulllist.cfm).

To understand the aquatic food chain itself, and problems associated with reliable research in this area, is explained in broader detail in Pennak ...

Page 32, Pennak's Freshwater Invertebrates of the United States
"During the past two generations the literature of aquatic biology has been replete with "productivity" and "trophic level" studies. It is unfortunate that most of these studies are misleading because they (1) are seasonal rather than year round, (2) have been conducted on complex (rather than simple) ecosystems, and (3) are based on only a fraction of the species making up a particular ecosystem. As a consequence, the generalizations postulated in many of these investigations are open to doubt and modification. Actually, a much more fruitful approach of population dynamics and interrelationships involves habitats characterized by few species and few niches. Examples are alkali and saline ponds, relatively barren alpine lakes, spring brooks, very small ponds, warm springs and small mountain streams. Using such bodies of water, it should be relatively easy to determine food habits of individual species and to make quantitive estimates of populations. Productivity indexes, food cycles, energy transfers from one trophic level to another, species interactions, and seasonal population fluctuations are all important ecological concepts that should be derived from such studies on a year-round basis. It is important to note that the measurement of associated physical and chemical factors in small streams, lakes, and ponds is simpler than in more complex types of aquatic habitats and in terrestrial habitats.

Fig. 1.12 Basic features of aquatic food webs. See text for explanation.
Nevertheless, even in the simplest aquatic habitats the determination of pyramids of numbers and food chains (or, better, "food webs") is an involved problem, and only a very few thorough and inclusive studies have been attempted. The great majority of trophic investigations are concerned with only a few energy or biomass transpositions, the remainder being only roughly estimated. In any such study the basic role played by detritus [that is, loose fragments or grains worn away from rock and/or disintegrated, eroded matter] and bacterial action should not be minimized. In a schematic fashion, Fig. 1.2 shows the fundamental food interrelationships for any aquatic habitat, regardless of size and complexity. Only recently have aquatic ecologists come to realize the overwhelming role of detritus in the food web. The significance of various segments of this diagram vary enormously from one habitat to another. Some habitats, for example, have few or no fishes; other habitats have no rooted aquatics or a poor substrate fauna.
Solid lines indicate consumption or utilization of food. Broken lines indicate death and disintegration of detritus as the result of cetain bacterial activities. The circle arrows originating and ending at "zooplankton," "fish," and "substrate fauna" are meant to show that certain species in each of these categories are carnivorous and feed on other individuals within the same category. Actually each such category may represent two, three, or more trophic levels. "Substrate fauna" includes all bottom invertebrates as well as protoctistans, micrometazoans, and macrometazoans associated with the surface of rooted aquatic plants and other objects. By inference, "bacteria" also includes molds and their activities.
For simplicity, certain minor aspects of this food web have been ommitted. For example, the contribution of excretory materials to the main reservoir of dissolved nutrients is not shown. By inference, "fish" should include other vertebrates, such as frogs, salamanders, and turtles. Not shown, but nevertheless intimately concerned with the continuing balance within the web, are the nitrogen, sulfur, and phosophorus cycles. These are fundamentally similar to the same cycles operating in terrestrial habitats."
Source: Pennak's Freshwater Invertebrates of the United States, pg. 32-33

yep, Pennak is a great reference for freshwater inverts.. I have a 1978 edition on my shelves -- haven't used it in awhile since I really don't do FW stuff anymore. I just pulled it out and checked out the intro.
You're right -- it's a good discussion on the FW food web.

Enjoy your new treasures!

Terri
www.ncseagrant.org