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Figure 2. Summary data for 5 cohorts of fathead minnows identified and incorporated into the bioenergetics model. The data were collected in 1997. Panel (A) is average lengths of fish in each cohort. (B) and (C) are densities of fish per ha in each cohort, and (D) is biomass per ha for each cohort. Notice Y axis differ between (B) and (C).

Integrating the results from the five cohorts into population totals shows a gradual decline of fish density from 19 May through 23 June, but then a rapid increase due to production of YOY fish (Figure 3A). Total biomass also shows a gradual decrease until 23 June, when it increases rapidly and reaches a peak in late July (Figure 3B). Phosphorus and nitrogen stored in the fish population (as tissue, bone, etc.) follow the same pattern as biomass, again reaching a peak in late July (Figure 3C). Nitrogen and phosphorus releases from fish mortality were substantial, particularly after 3 June. These releases were very dynamic, with peaks associated with the high mortality of cohort 1 from 3 June to 23 June, as well as peaks associated with the initiation and subsequent mortality of YOY cohorts after 23 June. Mortality during the period of 3 June to 23 June represented a substantial loss of nutrients from the fish pool, amounting to a daily loss of ~5% of the nitrogen and phosphorus in the fish biomass pool.

Figure 3. Total numbers (A), biomass (B), amount of nutrients contained in population (C), and amount of nutrients lost from fish pool due to mortality (D). Data are integrated results for 5 cohorts of fathead minnows sampled in 1997. Notice Y axis differ between nitorgen and phosphorus for (C) and (D).

We found nine different types of prey in the stomachs of fathead minnows and observed a greater diversity of prey in May and June than in July and August (Table 2). Overall, the diet composition varied considerably through time. Copepods, chironomids, and algae were the most important prey items; copepods were most important in June, chironomids in May and July, and algae in August. The algae in the diet appeared to be mainly cyanobacteria similar to what we observed growing in mats on the bottom of the wetland. The importance of chironomids, copepods, and algae in the diet is reflected in the bioenergetic model estimates of total consumption during the study period. Consumption was highest for chironomids (412 kg•ha^-1), copepods (279 kg•ha^-1), and algae (169 kg•ha^-1) (Table 3). These prey items were also the most significant sources of nitrogen and phosphorus. Overall fathead minnow consumption of invertebrate prey was 784 kg•ha^-1, and this represented a substantial loss of nutrients from invertebrate pools, with 10 kg•ha^-1 of nitrogen and 0.8 kg•ha^-1 of phosphorus consumed.

Time-specific consumption rates of copepods, chironomids, and algae showed copepods and chironomids to be most important from 19 May through 22 July, but from 22 July to 4 August these decreased in importance as algae increased (Figure 4A). Overall, consumption rates for copepods and chironomids increased substantially on 23 June, which corresponds to the appearance of the first YOY cohort (cohort 3). The change in consumption rates on 22 July represents the substantial change in diet of fathead minnows, in essence the late-summer switch from invertebrate to algal prey (see Table 2). Total consumption expressed in terms of phosphorus and nitrogen showed a decreasing trend when the population was composed only of overwintering fish (19 May through 23 June), but then a rapid increase upon appearance of YOY fish (23 June through 4 August) (Figure 4B). Changes in the relative slopes of the phosphorus and nitrogen data in Figure 4B reflect changes in the diet and the nutrient concentrations of prey. The most pronounced change in relative slopes occurs from 22 July to 4 August, when the fish switched from consuming invertebrate prey to algae. The change in the nitrogen slope is due to the much lower nitrogen:phosphorus ratios of algae relative to most invertebrate prey. Phosphorus and nitrogen allocated to growth showed a trend similar to consumption, with decreasing values from 19 May through 23 June, and increasing values from 23 June to 4 August (Figure 4C). Again, this change corresponds to the appearance of YOY fish.

Figure 4. Consumption rates for three most important prey types (A), nutrient consumption (B), allocation to growth (C), and excretion (D) by fathead minnows in Sagebraten. Data are integrated results for 5 cohorts sampled in 1997. Notice Y axis scales differ between nitrogen and phosphorus for (B) and (C).

Our estimates of nitrogen and phosphorus excretion rates differed considerably (Figure 4D). Excretion of nitrogen was substantial but highly variable throughout the modeled time-period. In general, nitrogen excretion decreased from 19 May to 23 June, increased from 23 June to 22 July, and then decreased from 22 July to 4 August. After accounting for nitrogen lost through egestion, on average 61% of assimilated nitrogen was allocated to growth and 39% excreted during the time period when invertebrate prey dominated the diet (19 May to 22 July). When the fish switched to a diet composed mainly of algae (22 July to 4 August), on average 87% of nitrogen assimilated was allocated to growth and 13% was excreted. In contrast, 100% of assimilated phosphorus (consumed minus egested) was allocated to growth, as the model produced negative excretion values during the modeled time period (Figure 4D). This indicates that fathead minnows excreted no phosphorus during the study period. This result suggests that the amount of phosphorus consumed was below levels needed to meet the growth requirements of the fish, and so all consumed phosphorus was allocated to growth, leaving none to be excreted. It also indicates that growth of the fathead minnows in this wetland may be phosphorus limited.

Summarizing results of the bioenergetics model into a nutrient budget indicates the fathead minnow population had a net uptake of 2.62 kg•ha^-1 of nitrogen and 0.52 kg•ha^-1 of phosphorus during the modeled time period (Table 4). Total consumption was equal to 10.81 kg•ha^-1 for nitrogen and 0.94 kg•ha^-1 for phosphorus, and egestion was equal to 2.16 kg•ha^-1 and 0.26 kg•ha^-1 for nitrogen and phosphorus, respectively. Gross assimilation (total consumption minus egestion) was 8.65 kg•ha^-1 of nitrogen and 0.68 kg•ha^-1 of phosphorus. Of the assimilated nitrogen, 6.67 kg•ha^-1 was allocated to growth (77%), and 2.38 kg•ha^-1 excreted (28%). Of the assimilated phosphorus, 1.03 kg•ha^-1 was allocated to growth (151%), and 0 kg•ha^-1 excreted (0%), again indicating that ingested amounts of phosphorus were insufficient to meet growth demands, resulting in no phosphorus excretion. Total amounts of nutrients released from fish mortality were 3.44 kg•ha^-1 of nitrogen and 0.65 kg•ha^-1 of phosphorus. Thus, the amounts of nitrogen and phosphorus released via fish mortality were greater than the amounts excreted. Also worth noting is that the amounts of nitrogen and phosphorus released by fish mortality exceeded net uptake into the fish population. This indicates that nutrient retention in the fish population is short-term, and nutrients incorporated into fish tissue are rapidly recycled back into the water column via fish mortality.

Results of our sensitivity analysis showed that perturbations of +10% of original values resulted in less than 10% changes in estimates of phosphorus consumption and excretion (Table 5). Physiological variables that had the greatest influence were CTO, RA, RQ, RTO, ACT, and UA.