Cyclical Browning in Lake Annie

Research Focus: This study investigates the drivers of lake browning in subtropical Lake Annie, Florida, by analyzing diatom assemblages and their relationship with colored dissolved organic carbon (cDOC) and water transparency.

Climate Change: Alteration in climate and land use is leading to increased inputs of cDOC into lakes.
Browning: Increased colored dissolved organic carbon (cDOC) decreases water transparency and alters lake physicochemistry with biological consequences.
Management Implication: Understanding drivers of cDOC change is crucial for adaptive watershed management and mitigating climate change impacts on aquatic ecosystems.

Objective: To determine the effects of cDOC on diatom assemblages and lake transparency in Lake Annie through paleolimnological methods and long-term monitoring data.
Approach: The study combined long-term limnological monitoring data (14-year dataset) with paleolimnological analyses (sediment core) to evaluate the relationship between cDOC inputs and diatoms.
Transfer Function R2 Value: A strong transfer function was discovered between historical diatom assemblages and water transparency, with an R2 value of 0.72.

The % composition of diatoms are effective indicators of past water quality and cDOC dynamics - this is because diatoms are sensitive to changes in water chemistry and light availability - changes in their % composition reflect changes in cDOC levels (cDOC levels directly impact water chemistry and light availability).
Recent studies observed significant shifts in water transparency due to increased cDOC levels, implicated in the reduction of lake water quality.
Current Observations: Approximately 18% reduction in “blue” lakes in the U.S. from 2007 to 2012 attributed to cDOC impacts (Leech et al., 2018).

Browning affects lake transparency and alters thermal regimes (temp layers in the lake), leading to greater hypolimnetic anoxia (deeper levels of the lake have reduced oxygen levels), impacting lake biota distribution (many species rely on oxygen to survive).
Changes in plankton community dynamics tied to nutrient loading and light penetration vary between clear and brown conditions (Kelly et al., 2018).

Lake Annie has a history of oscillation between clear and brown states influenced by anthropogenic modifications (e.g., ditching - constructing ditches or channels that are designed to drain or alter waterflow - they can increase inputs of cDOC in lakes) alongside climate variability (Gaiser et al., 2009).

Type: Subtropical sinkhole lake located in Highlands County, Florida.
Size: 0.36 km²; maximum depth of 21 m.
Monitoring History: Monitoring began in 1983, with specific interest in water transparency and nutrient levels over time.

Monthly transparency evaluation began in 1983, with phytoplankton net tows added in 2005.
Nutrient Concentrations: Surface water total phosphorus concentrations ranged from 3 to 26 µg/L.

Human Impact: Modifications including ditch construction began in the early 20th century (1917-1930), affecting hydrology and cDOC inputs.
Hydrology Shifts: Groundwater remained a prominent water source, and modifications intensified impacts of climatic variability on water transparency.

Physicochemical Data Set: Monthly measurements from 2005-2019.
Annual Average Physicochemical Data Set: Used for historical trends from 1985-2019.
Sediment Core Data: A 166-cm core providing information over approximately 400 years.

Monthly samples collected using phytoplankton nets to determine seasonal variability in diatom assemblages.
Wet subsamples analyzed for diatom community composition using microscopy, identifying at least 600 diatom valves per sample.

Secchi Depth Observations: Monthly Secchi depth ranged from 0.7 to 6.7 m, reflecting significant seasonal variability correlating with hydrological inputs and cDOC concentrations.

Taxonomic Breakdown: 203 diatom taxa identified across two phases (clear and dark) correlated with fluctuating cDOC levels and transparency.
- Dark Phase Indicators: Asterionella ralfsii var. americana, Aulacoseira herzogii.
- Clear Phase Indicators: Brachysira brebissonii, Frustulia pseudomagaliesmontana.
Diversity Metrics: The Shannon Diversity Index indicated greater diversity during clear phases.

clear phase → ↓ cDOC → ↑ light penetration → ↑ habitat (benthic + pelagic) → ↑ niche availability → ↑ species coexistence → ↑ diversity

dark phase → ↑ cDOC → ↓ light → limited habitat → dominance by few tolerant species → ↓ diversity

Diatom Imperial Transparency: Estimated Secchi depths derived from sediment core data reflected historical clarity, with varying patterns in nutrient accumulation rates reflecting watershed changes.
Nutrient Dynamics: Trends in total carbon, nitrogen, phosphorus accumulation rates showed significant fluctuations tied to watershed modifications starting around the 1930s.

Browning in Lake Annie exemplifies how local modifications can amplify the impacts of climate-induced changes in aquatic ecosystems.
Future Management Recommendations: Suggests a need for focused watershed management to counteract cDOC input and maintain ecosystem health during warming periods, especially during AMO oscillations.

Continued long-term meteorological and limnological monitoring to better understand interactions between climate and land use on lake browning and improved protection measures for aquatic ecosystems.

Numerous references are provided concerning ecological studies, lake management, and historical climate data relevant to Lake Annie. [Refer to original text for detailed citation.]

This research is supported by various institutions including the Archbold Biological Station and consists of contributions from multiple researchers and institutions, ensuring comprehensive data analysis and model validation.

Datasets are available publicly, contributing to transparency in research and further opportunities for study.