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Botulism outbreaks occurred in sentinel mallards in 2 wetland enclosures (P-2, P-8) in both 1987 and 1989 (Fig. 1); no outbreaks were detected in 1988. During these botulism outbreaks, weekly mortality rates in sentinel mallards ranged from 1.0 to 8.8 deaths/100 birds. Although botulinum toxin was found in 2 carcasses in another enclosure (T-F) in 1988, these cases were widely separated in time (>2 months), and botulism was not detected in wild birds in this or any other wetland in 1988. Because our criteria for classifying outbreak wetlands for this study required the detection of >1 carcass with botulinum toxin, the cases in T-F in 1988 were not considered to constitute an outbreak.

The range and variability of wetland conditions in our 4 study sites (means, minimums, maximums) fell within expected ranges (Table 1). Water temperatures varied from 13.0 to 26.8°C, with sediment temperatures generally 1–2 degrees higher. The water was very fresh, with specific conductivity values in the range of 88–896 µmhos/cm, and water pH was neutral to moderately alkaline, within a range of 6.0–8.4. Invertebrates included in our statistical analysis (counts and benthic biomass) included gastropods, nematodes, oligochaetes, eubranchipods, lymnaeid snails, cyclopoid copepods, ostracods, unknown dipterans, ceratopogonids, chaoborids, culicids, tipulids, ephydrids, notonectids, muscids, stratiomyids, unknown chironomids, and chironomids of the genera Goeldichironomus, Glyptotendipes, Tanypus, and Procladius. Other invertebrates had very low biomass or counts and were only included in total biomass calculations.

Table 1. Environmental variables measured and averaged (no. of sampling intervals [n], mean [], minimum [min] and maximum [max]) in 1.6-ha wetland enclosures where botulism outbreaks occured (outbreak wetlands) and did not occur (nonoutbreak wetlands) in sentinel mallards at the Sacramento National Wildlife Refuge, California, 1987-89. Outbreak wetlands were further separated into outbreak intervals (i.e., when botulism outbreaks occured in sentinel mallards) and nonoutbreak intervals (i.e., when no outbreaks occured). For each sampling interval, environmental measurement averaged from 5 locations distributed throughout the wetland enclosure included redox potential (redox) and standardized redox potential (stand. redox) measured in millivolts, temperature measured in °C, specific conductivity measured in µmhos/cm, pH, numbers of benthic invertebrates (benthic counts; log transformed), mass (g) of benthic invertebrates (benthic biomass; log transformed), mass (g) of total invertebrates (total biomass; log transformed), dissolved oxygen in the water measured in mg/L, percent organic matter in the sediments, water depth measured in centimeters, and water turbidity measured in nepholometer turbidity units.

	Outbreak wetlands (P-2, P-8)
	Outbreak intervals				Nonoutbreak intervals				Nonoutbreak wetlands (T-F, T-19)
Variable	n		min	max	n		min	max	n		min	max
Redox Wa	15	208	50	338	40	196	-26	467	56	219	8	477
Redox S1b	15	111	-24	303	40	19	-171	254	56	80	-80	337
Redox S2c	15	62	-40	256	39	47	-165	289	56	49	-174	375
Stand. redox W	15	251	86	367	40	227	16	505	56	248	17	485
Stand. redox S1	15	109	-43	252	40	28	-177	274	56	86	-65	317
Stand. redox S2	15	61	-43	252	39	47	-165	289	56	63	-160	359
Temperature W	15	22.4	18.3	26.5	41	20.6	13.0	26.6	57	20.4	12.1	26.8
Temperature S1	15	23.9	19.4	28.3	40	22.1	13.5	28.0	56	21.7	13.7	27.6
Temperature S2	15	23.8	19.5	27.6	39	22.1	13.9	28.0	56	21.8	14.4	27.3
Conductivity W	15	474	285	882	41	428	231	896	57	336	88	864
Conductivity S1	15	793	584	1086	40	733	530	1018	56	670	222	1186
Conductivity S2	15	1028	745	1275	39	1020	688	1510	56	1140	679	1819
pH W	15	7.73	7.09	8.80	41	7.54	7.00	8.77	57	7.51	6.38	8.75
pH S1	13	7.26	7.00	7.47	38	7.26	6.75	8.03	53	7.19	6.50	8.35
pH S2	13	7.14	6.94	7.53	37	7.30	6.73	8.02	53	7.28	5.99	8.87
Benthic counts	15	1.68	1.24	2.15	38	1.53	0.83	2.41	55	1.72	0.87	2.64
Benthic biomass	15	0.010	0.001	0.042	38	0.006	0.000	0.049	55	0.005	0.00	0.02
Total biomass	15	0.012	0.003	0.045	38	0.009	0.002	0.051	55	0.008	0.00	0.08
Dissolved oxygen	15	3.94	1.51	7.05	41	4.20	0.92	10.32	57	4.20	1.00	10.05
% organic matter	15	12.1	4.5	22.5	39	10.8	4.7	18.3	57	6.1	3.8	7.8
Depth	15	52.5	35.8	100.8	41	53.6	22.8	105.7	57	53.8	34.8	85.6
Turbidity	15	11.4	2.4	34.3	40	23.8	2.2	150.0	57	25.6	2.1	78.2
aW refers to water samples collected 7.5 cm above the sediment-water interface.
bS1 refers to interstitial water samples collected 2.5 cm below the sediment-water interface.
cS2 refers to interstitial wtaer samples collected 11.25 cm below the sediment-water interface.

Seven principal components identified by factor analysis accounted for 79% of the variation in the original 22 environmental parameters. Rotated factor loadings showed that environmental parameters in the soil and water were highly correlated (Table 2). The resulting 7 environmental factors had a relatively simple interpretation in relation to the original environmental parameters. Redox potential variables (REDOX) explained the largest portion of the variation (23%) found in the environmental measurements. Temperature variables (TEMP) and a factor consisting of specific conductivity and water pH (SC–pHW) accounted for an additional 26% of the variation. The fourth factor (pHSOIL) represented soil pH, and the fifth factor (INVERT) represented variables related to invertebrates. The sixth factor (DO–DEPTH–POM) was positively associated with dissolved oxygen and water depth but negatively associated with percent organic matter. Water turbidity was the only environmental variable strongly represented by the seventh factor (TURB).

Comparisons Between Outbreak and Nonoutbreak Wetlands

Comparisons Within Outbreak Wetlands

Preliminary examination of daily mortality rates during outbreaks indicated a larger range of weekly mortality rates in P-8 than in P-2 (Fig. 1). Therefore, correlations between mortality rates and the 7 principal factors were conducted separately for each enclosure. For P-2, daily mortality rates during botulism outbreaks (excluding Jul 1987 due to missing factor data) were negatively correlated with SC–pHW ( = -0.87, P = 0.02, n = 6) and INVERT ( = -0.90, P = 0.01, n = 6) factors. In contrast, daily botulism mortality rates for P-8 (excluding Jul 1987) were positively correlated with TURB ( = 0.83, P = 0.02, n = 7).