Trematodes are important parts of ecosystems and can have drastic bottom-up effects on biogeochemical cycling. At one of my study sites the trematode Cotylurus flabelliformis (snail→snail→duck) is very common and often reaches prevalence levels of 90% to 100% in ponds late in the season. Trematode infection alters the metabolism of the snail, changing the stoichiometry of snail excretion and snail grazing rates. In systems where snails dominate nutrient cycling and parasites dominate snail populations (as in this case), large effects from infection are seen in the water column and periphyton communities. Similarly, at another study site, the trematode Posthodiplostomum minimum (snail→fish→kingfisher) heavily infects bluegill fish at a state fishery. The parasite affects fish growth but also alters fish metabolism and excretion. Again the parasite provides a strong bottom-up control on biogeochemical cycling because of the prominence of fish excretion in freshwater nutrient budgets.
Inversely, there are also important top-down interactions between biogeochemical cycles and trematodes. Adding nutrients to an aquatic system has been shown to affect trematode populations and communities. Complex life cycle parasites (such as trematodes) seem to become more abundant with moderate levels of eutrophication. Nutrients can alter patterns in disease transmission directly through changes in snail intermediate host food quantity (grams of periphyton per area) and food quality (CNP periphyton stoichiometry). Nutrients can also affect snail habitat availability, reducing infected snail host mortality and raising parasite output per snail host. Nutrients may also reduce competition between susceptible and non-susceptible snail hosts, and in some cases, favor survival of the susceptible species over the non-susceptible species. The presence of non-susceptible snails decreases the infection rates of susceptible snails by drawing parasites away from susceptible snails to snails which cannot be infected (the dilution effect).