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The uncommon deepwater plant communities include 10 species of moss, two species of stoneworts, and two species of liverworts. These communities support endemic and native invertebrate communities that likely play an important role in processing nutrients and carbon at the bottom of the lake. Endemic species include deepwater stonefly, and two species of blind amphipod. Human and environmental drivers of distribution and abundance of deepwater plants are likely the same as those driving cultural eutrophication and changes in water transparency in the lake (i.e., suspended particles, atmospheric deposition, nutrient loading, urban development, and local/regional climate change). The introduction of nonnative species including mysid shrimp, warmwater fish, signal crayfish may drive plant bed density through direct consumption of plant material. Actions taken to improve water transparency are also likely to improve conditions for deep water plant bed spatial and depth extent. There is no regular monitoring of deepwater plant communities but monitoring of existing beds at regular intervals could provide useful information about the status and trend of known deepwater plant communities.
A dramatic decline in the occurrence of plants from shallow to deep waters during the last 50 years has occurred. Recent surveys confirmed the presence of the deepwater plants. Locations indicate patchy and variable plant mass depending on the location within one year. Regular samples have not been collected during the evaluation period and it is not possible to differentiate those changes that may be expected as result of natural population dynamics from those that are the result of external disturbance.
- Status
- Considerably Worse Than Target
- Trend
- Insufficient Data to Determine Trend
- Confidence
- Low
- Recent surveys of deepwater plants in Tahoe suggest that the populations may have declined by as much as 80 percent since they were surveyed in the early 1960s.
- The uncommon deepwater plant communities include 10 species of moss (Bryophyta:Bryophytina), two species of stoneworts (Charophyta), and two species of liverworts(Caires et al., 2013). These communities support endemic and native invertebrate communities that likely play an important role in processing nutrients and carbon at the bottom of the lake. Endemic species include deepwater stonefly (Capnia lacustra), and two species of blind amphipod (Stygobromus tahoensis and S. lacicolus) (Caires et al., 2013; Chandra et al., 2015). Since 1960 when the deepwater communities were first surveyed there have been dramatic declines in both deepwater plants and macroinvertebrate communities, with estimated declines in native invertebrate density reaching 80 percent to 100 percent (Caires et al., 2013). Various explanations have been offered for the decline of deepwater plants (Caires et al., 2013; Chandra et al., 2015). The first mechanism may be due to changes in light penetration resulting from eutrophication. Increases in nutrient and particle concentration in the water reduce the amount of light reaching deepwater plant communities lowering their production. The second mechanism is a change in the biological community due to invasions by mysid shrimp and signal crayfish. Mysid shrimp migrate daily, feeding on the bottom either directly on sediment carbon, invertebrates, or algae that is growing on the deepwater plants thus disturbing them (Chandra et al., 2015). Signal crayfish migrate seasonally with some crayfish living in or near deepwater plant beds during the summer. This results in direct predation on plant beds and the associated invertebrates. While it is not likely that invasive mysid shrimp populations can be controlled in the lake at this time, slow growing crayfish which live nine to 10 years may have the potential for control. Research could lead to quantification of spatial distribution and variability, a better understanding of the influence of crayfish on plants and endemic invertebrates, and the association between plant habitat and the life-history of endemic invertebrates.
- Human and environmental drivers of distribution and abundance of deep water plants are likely the same as those driving cultural eutrophication and changes in water transparency in the lake (i.e. suspended particles, atmospheric deposition, nutrient loading, urban development, and local/regional climate change). The introduction of nonnative species including mysid shrimp, warmwater fish, signal crayfish densities may drive plant bed density through direct consumption of plant material. Basic monitoring and research that experimentally moves plants from one location to another in the lake (e.g. plug and grow) and studies focused on understanding light limitation and endemic invertebrate life history association with plants is needed to quantify the dominant controls on deepwater plants.
No related projects or programs defined for this indicator.
- Technically unknown or unknowable due to insufficient data. Likely considerably worse than target. In the most recent survey the deepwater plant communities were not found in the many areas of the lake they historically occupied. The standard is a nondegradation standard, so the absence of plants in areas inside their historical range suggests there has been degradation. When the standard was adopted in 1982 the most recent extensive survey of the deepwater plants in Lake Tahoe was already over 15 years old (Frantz and Cordone, 1967). Because no baseline was established at the time the standard was adopted it is impossible to say at what point in the last 50 years the decline of deepwater plants occurred or if declines are continuing today.
- Insufficient data to determine trend. Due to limited sampling, there is insufficient data to determine trend. However, the magnitude of decline observed between the two sampling events is abnormally large for deepwater aquatic plant communities that generally have relatively stable population dynamics. Although the magnitude of decline cannot be accurately quantified because no estimates of biomass are available from the earliest surveys, the spatial extent of community decline suggests that there has been a rapid decline in the deep-water plant species over the last 40 years (Caires et al., 2013).
- Moderate. Regular samples have not been collected during the evaluation period. As a result, it is not possible to differentiate those changes that may be expected as result of natural population dynamics from those that are the result of external disturbance.
- Low. No trend assessment can be completed at this time because only two samples are available. However, the magnitude of decline observed between the two sampling events is abnormally large for deep-water aquatic plant communities that generally have relatively stable population dynamics (Caires et al., 2013).
- Low. Overall confidence takes the lower of the two confidence levels.
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