Stalked Diatoms

Periphyton

Around the world, many clear lakes are experiencing periphyton blooms, or quick increases of algae attached to the boundaries of lakes. In many lakes this is a fairly new phenomenon.  Periphyton is an indicator species, so when the amount of periphyton changes, it can be an early sign that the larger environment is changing. Little research has been done on periphyton blooms because they are difficult to monitor. Lake Tahoe has one of the longest and most detailed datasets about periphyton in the world. In order to better understand processes affecting periphyton, a UC Davis TERC Ph.D. Candidate, Karen Atkins, has several projects underway. First, she explored long term trends and variability in the Tahoe periphyton. She found that though there is lots of variability from year to year and site to site, overall, periphyton blooms have not increased in biomass since 1982.  Learn more about what she found here. Now Karen is using statistical modeling techniques to  understand the combined influences of light, nutrients, lake level, and bed-shear stress on periphyton. She is also using modeled climate change predictions to understand how Tahoe's periphyton will change in the future. Finally, Karen is leading a group of international scientists creating methods to monitor groundwater and periphyton together in order to strengthen our understanding of their impact on one another and their effects on lake ecosystems at large

Dissertation abstract:

Around the world oligotrophic lakes are changing. Once pristine shores are increasingly covered in dense mats of periphyton, known as filamentous algae blooms (FABs). Periphyton blooms occur in the infrequently monitored littoral zones of lakes, and thus are likely underreported and poorly understood. While some blooms have known causes, such as increased groundwater nutrient levels or as an influx of invasive species, many FABs occur for unknown reasons. This dissertation examines the drivers contributing to FABs. In three chapters, Lake Tahoe, CA/NV, is selected as a case study because, unlike many lakes, Tahoe has a long history of periphyton monitoring.

Overall, eulittoral periphyton biomass at Lake Tahoe did not shown an increase from 1982 through 2019. Periphyton was measured consistently at 0.5m depth at multiple locations. Thus, as lake level changed over a range of 2.5 meters, monitoring recorded the biomass of either the diatom and green algae dominated community that inhabited the near-surface substrate, or the cyanobacteria dominated community that thrived at greater depths. Trend analysis showed that the stalked diatom and green algae communities did not show a statistically significant change in biomass over time. However, the cyanobacteria measurements showed a significant decline in biomass over time.

The monitored and derived drivers of algae did not explain trends in Lake Tahoe periphyton biomass. Temperature, lake surface elevation, bed-shear stress, mid-lake nutrients, and photosynthetically active radiation did not explain most of the variation in periphyton biomass. Rough groundwater estimates made for the lake’s nutrient regulation explained more of the variability. This is not surprising, as studies have shown that groundwater nutrients are correlated with periphyton biomass in Lake Tahoe. Oligotrophic lake managers must monitor groundwater nutrients along with periphyton biomass levels.

Looking toward the future, climate change model results were analyzed to project periphyton community and biomass shifts over time in Lake Tahoe. Local climate change conditions will affect many of the drivers of periphyton, including warmer lake temperatures, more nutrient inflows, and greater variability in lake levels. Combining the many changes, two potential outcomes are offered. In one scenario, diatoms continue to dominate the euphotic zone of periphyton, but biomass increases. In the other scenario, an increase in nutrients and temperature shifts the lake towards a green algae dominated euphotic zone. In both scenarios, cyanobacteria are likely to remain the dominant periphyton in the sublittoral zone. These findings suggest that in addition to periphyton drivers, periphyton biomass and community composition should be continually monitored in Tahoe to understand potential change.

Despite the need for more ecohydrological information, few studies analyze groundwater and periphyton and even fewer programs consistently monitor groundwater and periphyton together. Suggestions are made to overcome the barriers created by underlying geology, intra-lake and cross-lake landscape processes, benthic biological systems, and data gaps. Trusted and new methods for periphyton and groundwater monitoring are reviewed, and recommendations are proposed for integrating these methods. Using these combined methods, and the combined knowledge of hydrologists and ecologists , we can further hydrology and ecology.