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Figure 1. Sedimentation rates of phosphorus and nitrogen in Sagebraten and Rollag in shallow and deep strata (+ 1 SE).

Figure 2. Interstitial-water concentrations of phosphorus and nitrogen in Sagebraten and Rollag in shallow and deep strata (+ 1 SE).

Different patterns of interstitial phosphorus concentrations were observed between the two wetlands throughout the summer (Figure 2, top two panels). Low concentrations were evident in both wetlands on 19 May (both strata), but concentrations increased markedly in Rollag while remaining low in Sagebraten as the summer progressed. In contrast, interstitial concentrations of nitrogen in the shallow stratum were similar in both wetlands and remained relatively constant throughout the summer (Figure 2, third panel). In Sagebraten nitrogen was lower than in Rollag in the deep stratum on 19 May and 23 June, but increased on 4 August to approximately the same concentration as Rollag (Figure 2, fourth panel). Sediment concentrations of phosphorus and nitrogen were similar in both strata of both wetlands on all dates (Figure 3), the largest differences we observed were nitrogen concentrations in the shallow strata, with higher levels evident in Rollag.

Figure 3.Concentration of nitrogen and phosphorus in sediment of Sagebraten and Rollag in the shallow and deep strata (+ 1 SE). Values are for cross section from sediment surface to 3 cm below surface.

Figure 4. Turbidity, phytoplankton, total phosphorus, and total nitrogen concentrations in water column for Sagebraten and Rollag.

Water-column characteristics differed greatly between wetlands. Water clarity was much lower in Sagebraten on all dates, with turbidity gradually increasing in Sagebraten while decreasing slightly in Rollag as the summer progressed (Figure 4, top panel). Phytoplankton biomass was also consistently higher in Sagebraten and peaked in August, while phytoplankton levels were consistently low in Rollag in the latter half of the summer (Figure 4, second panel). Water column concentrations of total phosphorus were similar in May, but then increased rapidly in Sagebraten while decreasing slightly in Rollag (Figure 4, third panel). Concentrations of nitrogen in the water column of Sagebraten were much more dynamic and differences were not nearly as pronounced as total phosphorus (Figure 4, bottom panel).

Macrophyte biomass in Rollag was much higher on all dates (Figure 5, top panel). Abundance increased dramatically from 19 May to 23 June, and then remained constant in Rollag, while abundance remained at consistently low levels in Sagebraten. Nearly all macrophyte biomass in Rollag on 19 May was contributed by a bed of Chara in the deep stratum. The biomass increase in Rollag on 23 June resulted from development of submersed species such as Potamogeton pectinatus in the mid-depth and deep strata, and emergent species such as Typha latifolia in the shallow stratum. In contrast, the slight increase in macrophyte biomass in Sagebraten between 19 May and 23 June was due mainly to development of emergent species in the shallow stratum. Periphyton biomass remained relatively constant and much higher in Rollag than in Sagebraten; no periphyton was detected in Sagebraten until 4 August (Figure 5, second panel). Metaphyton was more similar between wetlands than was either periphyton or macrophyton biomass, being relatively low in both wetlands (Figure 5, third panel).

Figure 5. Dry-weight biomass of macrophyton, periphyton, metaphyton, and aquatic invertebrates (+ 1 SE) in Sagebraten and Rollag. Note that Y axes differ between variables.

Figure 6. Numbers and biomass of fathead minnows and total phosphorus and nitrogen in fathead minnows (+ 1 SE) in Sagebraten.

Invertebrate biomass was over seven times higher in Rollag, compared to Sagebraten, on 19 May (86 kg per ha and 12 kg per ha, respectively) (Figure 6, fourth panel). By 23 June invertebrate biomass had increased 19% in Rollag (103 kg per ha), but remained unchanged in Sagebraten (12 kg per ha). Invertebrate biomass dropped considerably in Rollag on 4 August (46 kg per ha), but still remained much higher than in Sagebraten (1 kg per ha).

Adult fathead minnows in Sagebraten were most abundant 19 May and 3 June, declined dramatically by 23 June, and remained relatively constant during the remainder of the summer (Figure 6, top panel). Juvenile fathead minnows were least abundant on 19 May and 3 June, increased slightly on 23 June, and increased dramatically by 4 August (Figure 6, second panel). The net effect of these shifts was nearly constant biomass of fathead minnows until the last two sampling dates, when biomass increased exponentially (Figure 6, third panel). Thus, amounts of nitrogen and phosphorus contained in the fish pool remaining relatively constant until the last two sampling dates, when they also increased dramatically (Figure 6, bottom panel).

Phosphorus•ha^-1 measured in all seven nutrient pools was 5-6 fold lower in Sagebraten on all three sampling dates (Figure 7, top panel). Totals increased for both wetlands from the first to second sampling dates, but disparities between wetland sites remained similar. Comparing relative proportions of phosphorus in each pool reveals sharp contrasts in nutrient partitioning between wetlands. In Rollag the most important pools were macrophytes, periphyton, and aquatic invertebrates. On 19 May, the largest phosphorus pools in Rollag (as percent of wetland total) were macrophyton (63%), periphyton (18%), aquatic invertebrates (14%), and other seston (3%) (Figure 7, top panel). As the summer progressed macrophytes increased in percentage, while periphyton, aquatic invertebrates, and seston decreased. In Sagebraten on 19 May, the largest phosphorus pools were phytoplankton (41%), fathead minnows (27%), seston (19%), and aquatic invertebrates (11%). Seasonal changes in sizes of nutrient pools were much more dynamic in Sagebraten, with seston (53%), macrophyton (24%), phytoplankton (12%), and fathead minnows (7%) most important on 23 June, and fathead minnows (46%), seston (23%), macrophyton (17%), and phytoplankton (13%) most important on 4 August (all Figure 7, top panel).

Similar relationships were evident for total nitrogen in all measured pools. Nitrogen•ha^-1 was also higher in Rollag on all dates, and in both wetlands increased from the first to the second sampling date (Figure 7, bottom panel). On all dates, nitrogen•ha^-1 in all pools was only 2½ larger in Rollag than in Sagebraten. Nitrogen partitioning also differed greatly between the wetlands. On 19 May, the largest nitrogen pools in Rollag were macrophyton (46%), periphyton (21%), seston (20%), and aquatic invertebrates (11%). As with phosphorus, the percent in macrophytes increased as the summer progressed, while decreasing in periphyton, seston, and aquatic invertebrates. On 19 May the largest pools in Sagebraten were seston (76%), phytoplankton (14%), fathead minnows (5%), and aquatic invertebrates (4%). Seston remained the largest pool, with macrophyton second most important on 23 June and fathead minnows on 4 August (all Figure 7, bottom panel).

Figure 7. Results of phosphorus (top panel) and nitrogen (bottom panel) partitioning of the seven nutrient pools in Sagebraten and Rollag on 19 May, 23 June, and 4 August of 1997.

Figure 8. Results of phosphorus (top panel) and nitrogen (bottom panel) partitioning for Sagebraten and Rollag on 19 May, 23 June, and 4 August of 1997. Results are for six nutrient pools, macrophytes having been removed.

When macrophytes are removed from the total nutrient pool, the total nitrogen and phosphorus pools remain relatively constant in Rollag, but increase sharply in Sagebraten (Figure 8). This indicates that nutrients in Sagebraten are accumulating in the "open-water" nutrient pools of seston, phytoplankton, and fathead minnows. In contrast, nutrients in Rollag accumulated in macrophytes, periphyton and invertebrates remain important but constant, and the open water pools of seston and phytoplankton are consistently small.