RESULTS

Diaptomid Copepod Distribution

Five species of diaptomid copepods were found in high-elevation lakes in NOCA. Three of these species, Diaptomus kenai, D. arcticus, and D. tyrrelli, were relatively common and were pigmented red. The other species, D. lintoni and D. leptopus, were found only in a few low elevation lakes. Analyses were restricted to the three most common species. Chaoborus rarely occurs in NOCA high lakes.

Diaptomus kenai (mean length = 1.88 mm) and D. arcticus (mean length = 2.04 mm) are the largest zooplankters in NOCA. Diaptomus kenai is widely distributed among NOCA lakes while Diaptomus arcticus is much less common (Table 4.1).

Lakes with only large diaptomids (D. kenai and D. arcticus) were principally higher elevation (1243 m) lakes (Table 4.1). Diaptomus arcticus and D. kenai co-occurred in some of these lakes. The small herbivorous (Olenick 1983) copepod, D. tyrrelli (mean length = 1.18 mm), occurred allopatrically, and sympatrically with large diaptomids, across the entire range of lake elevations (Table 4.1).

Abiotic Factors

The density of D. kenai was not significantly correlated with any abiotic factor (P> 0.05). In contrast, the density of D. tyrrelli had a significant positive correlation with total Kjeldahl nitrogen (TKN; r = 0.54, P = 0.0036) and total phosphorus (TP; r = 0.53, P = 0.0044). Correlations of abiotic factors with D. arcticus abundance were not determined because the number of lakes with this species was small.

The small copepod was found only in shallow lakes (<= 10 m) that maintained high epilimnion concentrations of TKN (>= 0.05 mg l^-1; Figure 4.1) and TP (>=0.007 mg l^-1; Figure 4.2). In contrast, both deep and shallow lakes with low TKN and TP supported only D. kenai. Diaptomus kenai also occurred allopatrically and sympatrically with D. tyrrelli in shallow lakes with high TKN and TP.

Results of comparisons of abiotic factors among lakes with only large diaptomids, lakes with only the small diaptomid, and lakes with both large and small diaptomids were consistent with the correlation analysis. Lakes with only large diaptomids were significantly lower in TKN, TP, and conductivity than were lakes with only the small diaptomid (Table 4.2; Kruskall-Wallis, P = 0.0034, 0.0009, and 0.017, respectively). Lakes with only large diaptomids also were significantly lower in TKN and TP than were lakes with both large and small diaptomids (Table 4.2; Kruskall-Wallis, P = 0.0033 and 0.0009, respectively). However, there were no significant differences in any abiotic factor between lakes with only the small diaptomid and lakes with both large and small diaptomids (Table 4.2; Kruskall-Wallis, P> 0.017). Lakes with the small diaptomid (a group composed of lakes with only the small diaptomid and lakes with both large and small diaptomids) had significantly higher TKN, TP, pH, alkalinity, and conductivity than lakes where the small diaptomid was absent (Kruskall-Wallis, P 0.012).

Table 4.2. Mean and range (in parentheses) of abiotic factors in lakes with only large diaptomids, laks with only small diaptomids and lakes with both large and small diaptomids in North Cascades National Park Service Complex, WA, USA.

Copepod Relationships

Relationships between densities of large and small diaptomids was assessed using only lakes with nutrient levels within the range of occurrence of D. tyrrelli (TKN 0.05 mg l^-1, TP 0.007 mg l^-1; N = 17, Figures 4.1 and 4.2). For this set of lakes, we determined if TKN, TP, and large diaptomid density were related to D. tyrrelli density in each lake category (i.e., large diaptomids only, small diaptomids only, large and small diaptomids) using a general linear regression model.

For lakes with TKN 0.05 mg l^-1; and TP 0.007 mg l^-1; the slopes of the relationships between D. tyrrelli and TKN and TP were not significantly different from zero (P> 0.05) for each lake category nor were there any significant differences in mean values of TKN and TP between lake categories. Thus, although TKN and TP may have influenced whether D. tyrrelli was present in lakes, TKN and TP had no statistically discernible effect on D. tyrrelli density in lakes where their concentrations were 0.05 mg l^-1; and 0.007 mg l^-1, respectively. For lakes with both large and small diaptomids, there was a significant negative relationship between the logarithm of D. tyrrelli density and large diaptomid density (P = 0.0003, B in Figure 4.3). Diaptomus tyrrelli densities were highest in lakes where large copepods were absent (S in Figure 4.3).

Fish Predation

Lakes with high fish densities all supported reproducing populations of trout (Table 4.3). In these lakes, trout density was >450 fish ha^-1 in all lakes except Kettling Lake. Fish density was relatively low in only one lake with reproducing trout (MR 16). This lake was placed in the low fish density category. Except for MR 16, trout did not reproduce in lakes with low fish densities.

Table 4.3. Maximum summer densities of large copepods and Diaptomus tyrrelli in lakes with high fish densities, lakes with low fish densities, and lakes with no fish, North Cascades National Park Service Complex, WA, USA. Cutthroat (Onchorynchus clarki, Ct) and rainbow (Onchorynchus mykiss, RB) trout were present in the lakes with fish.

^aFish density estimated by mark-recapture and 95% confidence limits.
^bAverage density and range of fry stocked, determined from stocking records
^cFish density estimated by Leslie Method (Ricker 1975)
^dFishless lakes with salamander densities 5.0 larvae/100 m shoreline

Large diaptomid densities were significantly lower in shallow (maximum depth <10 m) lakes with high trout densities than in lakes with low trout densities (Table 4.3; Kruskall-Wallis, P = 0.0026). However, large diaptomid densities in lakes with high trout densities and in lakes with low trout densities did not differ significantly from large copepod densities in fishless lakes (Kruskall-Wallis, P = 0.045 and 0.07, respectively). A possible explanation for the lack of a significant difference in large copepod densities between lakes with high trout densities and fishless lakes is predation by salamander larvae on large copepods. With the exception of LS 1, salamanders were absent or low in abundance in lakes with fish in Table 4.3 (Liss et al. 1995; Tyler et al. 1998).

When fishless lakes with salamander densities 5.0 larvae 100m^-1 of shoreline were excluded from the analysis, there was a significant difference in large diaptomid densities between lakes with high fish densities and fishless lakes (Kruskall-Wallis, P = 0.0063). However, even when salamander lakes were excluded, there was still no significant difference in large copepod densities between lakes with low fish densities and fishless lakes (Kruskall-Wallis, P=0.42).

Fish density could be determined effectively by mark-recapture only in lakes with relatively small surface areas (<=7 ha) and shallow maximum depths (<= 10 in). To determine if lake depth influenced interaction of reproducing fish and large copepods we compared large copepod densities in deep lakes (maximum depth >10 m, N =6) with reproducing trout to those in shallow lakes. Large diaptomid densities were significantly higher in deep lakes (mean large copepod density = 0.35 l^-1, range = 0.03-1.04 l^-1) than in shallow lakes with reproducing fish (Kruskall-Wallis, P = 0.0004), suggesting that lake depth influenced the interaction between large diaptomids and reproducing fish.

Diaptomus tyrrelli was present, often at high abundance, in five of the seven lakes with high trout densities (Table 4.3). The small copepod was absent from most lakes with low trout densities and from most fishless lakes.

Further evidence suggesting that high densities of reproducing trout can lead to reduction or elimination of large diaptomids is provided by comparison of lakes connected by tributary streams. Rainbow Lake (Table 4.3) is the lowest in elevation of a series of four lakes. Small streams from the three higher elevation lakes, MR 13-1 (no fish), MR 13-2 (non-reproducing fish), and MM 11 (non-reproducing fish), flow into Rainbow Lake. MR 13-1 and MR 13-2 are only a few hundred meters from Rainbow Lake. Large copepods were relatively abundant in MR 13-1 and MR 13-2 (Table 4.3) and they were present in MM 11 (mean density = 0.44 l ^-1, sampled once in 1989 and 1993). Chemical and physical conditions in Rainbow Lake were suitable for large diaptomids (e.g., TKN = 0.040 mg l^-1, TP = 0.008 mg l^-1, maximum depth = 10 m) and yet the lake, which supports a dense population of reproducing trout, had virtually no large copepods (Table 4.3) despite ample opportunity for colonization from all of the three higher elevation lakes. Rainbow Lake was sampled three times in 1991, twice in 1989 and 1992, and once in 1993. Adult D. kenai were found at low density on only one of these sample dates.

Although Upper and Lower Panther lakes have a history of high fry stocking rates, D. kenai was relatively abundant in both lakes (Table 4.4). There was no statistically significant difference in large diaptomid density between Lower Panther Lake, which had been stocked with a high density of fry in fall 1990, and Upper Panther Lake, which was fish-free since fall 1990 (Table 4.4, Kruskall-Wallis, P> 0.99).

Table 4.4. Densities of trout and Diaptomus kenai in Upper and Lower Panther Lakes and number of samples per summer (N).

^aEstimated by Leslie method (Ricker 1975)
^bStocking density of fry
^cEstimated by mark-recapture: mean and 95% confidence limits

In Lower Panther Lake, large diaptomid density was higher in 1990 prior to fry stocking than in any subsequent year (Table 4.4). However, there was no significant linear time trend in large diaptomid density in the lake (P > 0.05). Mark-recapture estimates indicated that fish density in Lower Panther Lake had been reduced by nearly 60% by 1992 (Table 4.4), probably due largely to angler harvest. Although quantitative estimates of fish density in 1993 are lacking, observations while snorkeling indicate that trout density was low.

In Upper Panther Lake, which had been fish-free since fall 1990, there was a significant negative interannual trend in large diaptomid density (r = -0.42, P = 0.02) caused by a very high density in 1990. This was one of the highest densities of D. kenai observed during the study. A decline in D. kenai density is contrary to what would have been expected following trout removal if fish were preying heavily on the large copepod.