USGS Home
Contact USGS
Search USGS

Most studies of fish influences on phosphorus cycling have focused on excretion of phosphorus by fish (i.e. Carpenter et al. 1992b, Kraft 1992, He et al. 1993, Fuentetaja et al.1996). However, our results indicated that phosphorus regeneration from fathead minnow mortality was substantial and constituted the largest source of recycled phosphorus from the fish population. Few studies have assessed the influence of fish mortality on phosphorus cycling, but Threlkeld (1988) found that nutrient release from decomposing fish had a substantial affect on phytoplankton abundance in mesocosms. In what he referred to as the "dead fish paradox", Threlkeld found that fish mortality was associated with increased phytoplankton levels, while live fish reduced zooplankton abundance but had no effect on phytoplankton. Thus, decomposing fish stimulate algal production by releasing large amounts of nutrients. Additionally, Nakashima and Leggett (1980) estimated that perch mortality could account for 20% of the total phosphorus recycled in Lake Memphremagog. Our study indicates that the high turnover rate of fathead minnows results in increased turnover rates of nutrients, making these fish an influential component of the nutrient cycle in this wetland.

Our results do not change our general prediction that fathead minnows increase phosphorus recycling in wetlands, but they do change our ideas on the specific mechanism involved. We predicted that fathead minnows would have high consumption rates associated with high excretion rates, leading to high recycling rates. Instead, we found high consumption rates, low (zero) excretion rates, but high mortality rates. However, the effect on phosphorus cycling and the entire ecosystem is the same for both mechanisms, phosphorus is rapidly consumed from various nutrient pools, but its retention in the fish pool is short-term and the phosphorus is rapidly recycled into the water column. Thus, our data support the hypothesis that fathead minnow populations are associated with increased nutrient recycling, and this enhancement of nitrogen and phosphorus turnover rates is likely responsible for the increased levels of water-column phosphorus and higher algal biomass observed by Hanson and Zimmer (1998). Our estimate of zero phosphorus excretion and the suggestion of phosphorus-limited growth was unexpected, but can be explained by the characteristics of the fathead minnow population and their prey. Phosphorus-limited growth is uncommon but not unheard of in fish populations, and is most likely to occur in populations exhibiting high growth rates and consuming prey that are low in phosphorus (Schindler and Eby 1997). For example, Ketola and Richmond (1994) found that hatchery-reared rainbow trout exhibiting high growth rates were phosphorus-limited when fed food with phosphorus concentrations lower than 0.4%. Schindler and Eby (1997) examined 18 species of fish and found evidence of phosphorus-limited growth in only two species, both obligate planktivores feeding mainly on copepods (which have very low concentrations of phosphorus). Fathead minnows in Sagebraten exhibited relatively high growth rates, leading to high requirements of phosphorus for growth. However, model results indicated that consumption of phosphorus was inadequate to meet growth demands (indicated by the negative excretion values), suggesting that consumed prey were poor sources of phosphorus. The ability of prey to provide sufficient sources of phosphorus can be evaluated by the joule:phosphorus ratio (J:P) of the prey, with J expressed as joules•g wet biomass^-1 and P as µg phosphorus•g wet biomass^-1. Low ratios represent more phosphorus consumed per joule of prey, and indicate prey more likely to meet the phosphorus requirements of the fish as well as increase the probability of substantial phosphorus excretion. Calculated with data used in the model (data are from Cummins and Wuychek 1971, Hanson et al. 1997, Schindler and Eby 1997), ratios for the three most important types of prey in Sagebraten were 3.9, 3.4, and 2.1 for copepods, algae, and chironomids, respectively. For comparison, the ratio for Daphnia is 1.5 and 2.0 for amphipods. This indicates that, with the exception of chironomids, the prey consumed by these fish were poor sources of phosphorus. If this minnow population had consumed higher quantities of chironomids or other types of prey with low J:P ratios such Daphnia, it is likely that the population would have excreted substantial amount of phosphorus.

The diet composition of fathead minnows changed markedly from May through August. The diversity of prey was much greater in May and June compared to July and August, and this pattern probably results from decreased abundance of prey due to heavy predation by the fish. By July, copepods and chironomids were the only significant invertebrate prey, and their persistence in the diet is likely due to behavioral traits that make them less vulnerable to predation than other invertebrates. Chironomids are benthic organisms, and the sediment offers considerable refuge from predation. Copepods have strong, directional swimming and are thus much more effective at avoiding predators compared to most other crustaceans with different locomotion styles. However, it appears that by August predation by fathead minnows had also decreased the abundance of chironomids and copepods, and the fish were forced to rely heavily on algae as a food resource. Our suggestion that the change in diet is due to decreased availability of invertebrate prey is supported by the data in Zimmer et al. (1997), which shows a dramatic decline in invertebrate biomass in Sagebraten as the summer progressed. Our model results indicated that fathead minnows consumed large amounts of invertebrate prey (784 kg•ha-1 wet weight), suggesting these fish have dramatic impacts on the abundance of aquatic invertebrates. The impact of fathead minnow predation on aquatic invertebrates was also documented by Duffy (1998), who found that consumption of aquatic invertebrates by fathead minnows may approximate annual production rates of aquatic invertebrates.

Given that the degree of phosphorus excretion (and possibility of phosphorus limitation) by fish is heavily influenced by the type of prey consumed (Schindler and Eby 1997, Vanni and Layne 1997), fish species in which the diet composition varies from population to population will have differing effects on phosphorus recycling. Fathead minnows are opportunistic feeders, consuming a wide variety of prey (Held and Peterka 1974, Price et al. 1990, Duffy 1998). This indicates that the role of fathead minnows in phosphorus cycling is likely to vary from wetland to wetland, and will depend on the specific diet composition of each population. The relationship between diet composition and phosphorus excretion rates can be thought of as a gradient of low to high excretion, with excretion rates changing as the diet changes. At the low end of the excretion gradient the diet is largely copepods and algae, at moderate excretion rates the diet is largely macroinvertebrates, and at highest excretion rates the diet is largely cladocerans such as Daphnia. During the period of our study, minnows in Sagebraten functioned at the low end of this phosphorus excretion gradient, such that fish mortality was the only substantial source of regenerated phosphorus. However, if fish mortality is roughly the same in wetlands at both the low and high ends of the excretion gradient, fathead minnow populations feeding on phosphorus rich prey at the high end of the gradient will increase phosphorus cycling through two pathways; phosphorus excretion and phosphorus regeneration via fish mortality. These relationships predict that fathead minnows will have the largest influences on phosphorus cycling in wetlands where Daphnia are the most important prey, and also indicate that the fathead minnow population in Sagebraten likely had a minimal influence on phosphorus cycling. Further work is needed to examine the role of fathead minnows in nutrient cycling in populations consuming different types of prey, and to assess whether diet differences result in different effects at the ecosystem level.