Mono Lake Rotifers
This project was performed as an independent studies project with J.M. Melack, in the Department of Ecology, Evolution, and Marine Biology, at the University of California, Santa Barbara. Lab work was done at the Sierra Nevada Aquatic Research Laboratory, using the facilities of the Mono Lake Research Group. The research conducted during this project was later modified into the publication: Jellison, R., H.E. Adams, and J.M. Melack. 2001. Re-appearance of rotifers in hypersaline Mono Lake, California during a period of rising lake levels and decreasing salinity. Hydrobiologia. 466(1): 39-43.
Title: Rotifer Seeding of Mono Lake from Shoreline Ponds
Abstract:
Monitored rotifer populations of ponds located near Mono Lake, California, in summer and fall, 1999. Based on these populations of Brachionus plicatilis, calculations were made determining that it is feasible for pelagic rotifer populations to originate in shoreline ponds, and is likely given the previous research on rotifer osmoregulation and reproduction.
Introduction:
In October and December of 1997, Hexarthra jenkinae abundances of 18,000 m-2 and 101,000 m-2, respectively, were observed in Mono Lake (Jellison, 1997). However, since then, only lower abundances have been seen. Brachionus plicatilis appeared during the fall seasons of both 1998 and 1999 (Jellison, 1999) and may have been present before rotifers were noticed in Artemia monica samples from the lake. As the food size spectrum of rotifers is <1-20 mm (Arndt, 1993), increasing rotifer populations have the potential to be an important link between the traditional and microbial food webs within Mono Lake, feeding on algae, bacteria, and protozoans.
Mono Lake is a large, deep, hypersaline and alkaline lake. It has an area of 160 km2, a mean depth of 17 m, and a maximum depth of 45 m at 1,943 m elevation. Located east of the Sierra Nevada in California (38oN, 119oW), Mono Lake is a closed basin with the main hydrological input from rivers on the western side of the lake. Chemical composition of the lake consists mainly of sodium, chloride, and carbonate (Jellison and Melack, 1993).
In other high salinity lakes such as those found north east of Lake Chad, upper salinity limits for B. plicatilis have been found to be 70‰ and for H. jenkinae, 50‰ with maximum populations at salinities from 10-20‰ (Iltis and Duwait, 1971). According to Epp and Winston (1978), rotifers are osmoconformers which respond to any change in osmotic concentration by decreasing their metabolism. Due to the variable salinities found in Mono Lake, the rotifers are probably continually stressed from osmoregulation. Lubzens et al (1985) found no mixis, sexual reproduction of rotifers, at salinities greater than 35‰ and postulated that there would be a lower amount of energy allocated to reproduction due to the increased energy requirement of osmoregulation. Lower reproduction would therefore affect the number of resting eggs available for hatching within the lake.
The conditions necessary for hatching of B. plicatilis resting eggs have been studied by Minkoff et al (1983). Light was found to be the only obligatory cue for hatching, which would not be available to resting eggs in the center of Mono Lake. This result was confirmed by Hagiwara et al (1995) and may be due to light induced peroxide formation in salt water. Optimal temperature was 10-15oC, which is the surface temperature at which the large abundances of rotifers were found at Mono. Optimal salinity was 16‰ in the same experiment. The highest salinity tested was 40‰, at which over half of the eggs remained dormant and only approximately 15% hatched within the 11 day incubation period. However, these results were based on a single clone. Ito (1960) has suggested that the salt concentration during the formation of resting eggs is the optimal salinity for their hatching. Since no work has been done with rotifer resting eggs at Mono Lake, the hatching conditions can not be determined at this location. Due to the high salinity and lake depths at sampling stations where rotifers have been previously found, it is likely that the rotifers are being seeded from a location closer to shore.
In each of the three past
years, 1997-99, the surface elevation of the lake has declined about 1 ft
late in the year, rising only during January through July during snow fall
and melt (Jellison et al, 1999). During this period of falling
water depth, connected ponds immediately adjacent to the lake would drain
water and its contents into the lake. In order to test the hypothesis
that the pelagic rotifer populations are being seeded from shoreline ponds
during this period, various pools were sampled during summer and fall, 1999.
Methods
Ponds around the lake that were tested for rotifers are located on west, northeast, east, and south edges of the lake (Figure 1). Sites varied from being part of the shoreline, site 8, to being separated from the lake by about 200m of dry land, site 6. Most ponds were small in size and were separated from the lake by a strip of land ~5m wide. Site 5 was the largest pond sampled with a length of 200m, perimeter of 620m, mean depth of 1m, and has a 10m wide opening to the lake from its center.
In the field, 250mL of pond water was filtered through 120um sieves. The filtrate was kept to measure conductivities. The concentrated rotifers, algae, and detritus were rinsed into 60mL vials and all samples were refrigerated for transportation back to the research station. Upon return to the lab, 1mL of narcotizing agent, hydroxylamine hydrochloride 3% solution, was added prior to the addition of 1mL of 5% formalin to preserve the samples in a relaxed state. 3 subsamples of 1mL each were counted under 50x magnification with a stereoscope. Conductivities were measured with a conductivity meter, at room temperature.
Results
Figure 1 shows the location of sample sites around Mono Lake. There were no rotifers found at the County Park (site 8), South Tufa (site st), or sites 1-3. Highest abundances were found at sites 4, 5, and 6. Since site 6 was far from the lake, and unlikely to affect lake populations, only sites 4 and 5 were sampled multiple times.
At all sites, the major rotifer was B. plicatilis, with only 2 Hexarthra identified out of all of the samples. Most rotifers were found between 40 and 70 mυ as seen in Figure 2, a graph of rotifer abundances versus conductivity. Pelagic populations were present with salinity of ~80 g L-1 (Jellison et al, 1999).
A measurement of these populations over time was done at sites 4 and 5, on the south shore. Figure 3 shows these populations in September and November of 1999. This decrease, seen at both sites, was accompanied by a lowering water level of about 1ft in 2 months due to lack of rainfall that would offset evaporation. In addition, ice formation was observed on the surface of the ponds, particularly at the shallower site 4.
Discussion
Sites to the west had no rotifers, and lack of rotifers is echoed in lower abundances in the western pelagic zone of the lake (Jellison et al, 1999) and may be due to the influx of fresh water from nearby rivers. The very highest abundances were found separated from the lake by such a distance that the pond water is unlikely to wash into the lake, particularly with little to no rainfall as is characteristic for this region in the fall. Sites 4 and 5 showed a decrease in water level and some ice formation, suggesting the possibility that water drained into the lake, out of the pond. The decrease of rotifers in these 2 sites over the fall period accompanied an increase in the pelagic populations of rotifers. This increase in the lake during fall follows the trend of past years as seen in Jellison et al, 1999. There was also a greater increase in conductivity in these 2 sites in November, most likely a product of decreasing water levels and could be responsible for the lower rotifer populations due to increased stress of osmoregulation. According to Hofmann (1977), a time lag should be present between environmental change and response by rotifer populations. Heterogeneity between sampling sites was also evident, most likely due to slightly different abiotic conditions along the lake shore.
In order to determine if the shoreline ponds could support a large enough rotifer population to seed the rest of the lake at the found numbers, calculations were done comparing both the lowest and highest lake and pond concentrations of the fall period of 1999. Estimates of pond size were based on site 5, as were low and high rotifer densities, taken on 11/13/99 and 9/2/99, respectively. Lake low and high densities were used from station ET5.6 on 7/15/99 and 12/6/99 (Jellison et al, 1999). Calculations establish that lake rotifer populations are possible with approximately 45 ponds the size of site 5 in order to seed the entire lake for the largest population size of fall, 1999 with the lowest pond densities. However, the low lake population would be possible with less than one half of a pond the size of site 5, with either the low or high rotifer densities found in this study. These calculations are based on a one time seeding event of the entire pond population. The higher lake densities would be possible with connected ponds seeding the lake multiple times over the fall period when lake levels are lowering. This would cause a rotifer influx to the lake that would be dependent on rotifer regeneration times as well as water flow rates.
Conclusions
Previous rotifer research indicates that reproduction is unlikely within the pelagic zone of Mono Lake, this experiment has determined that shoreline pond populations are significant in size to make seeding of the lake possible. However, it is not know how long the time lag is between populations leaving the ponds and reaching the open water sampling stations within the lake. How fast the rotifers are able to replenish the pond populations, if at all, during decreasing water levels would make an interesting next step in this investigation. In addition, field observations indicated that some inland ponds between sampling sites were dry over the summer but may support rotifers in the first half of the year, generated from resting eggs, and may become an input as the lake level rises in the future due to decreased diversions by the LADWP, increasing the lake populations of rotifers in future years.
Literature Cited
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