Long-Term Hydrologic Fluctuations and Dynamics of Primary Producers in a Tropical Crater Lake

Aquatic ecosystems in tropical regions remain understudied and their long-term dynamics poorly understood. In East Africa, a better understanding of how natural communities of primary producers in small freshwater ecosystems respond to climatic variability is needed to improve management and conservation of aquatic resources. This study explored the response of algae and bacteria communities to marked hydrological variation over the past 1,500 years in a small western Ugandan crater lake, Lake Nkuruba. We analyzed sedimentary algal and bacterial pigments to evaluate the magnitude and direction of change in the autotrophic community in response to severe climatic perturbations in the region. The lithology of the Lake Nkuruba sediment core indicated that external forcing in the form of a major drought, associated with the Medieval Climate Anomaly, caused a heavy, short-lived detrital pulse to the basin that led to a brief but substantial disruption of the lake system in the second half of the Thirteenth century. The system appears to have recovered rapidly, and then transitioned to a more productive state than the one preceding the drought. The considerable variation observed in the sedimentary pigment biomarkers is likely linked with climatically-induced changes in the water column structure of this small crater lake. Our results highlight the challenge of defining appropriate baselines or reference conditions in climatically-sensitive East African aquatic ecosystems and disentangling long-term anthropogenic impacts from the strong regional hydrological flux at the decadal to centennial scale

Small changes in climate can profoundly alter the dynamics and ecosystem services of tropical crater lakes.

African tropical lakes provide vital ecosystem services including food and water to some of the fastest growing human populations, yet they are among the most understudied ecosystems in the world. The consequences of climate change and other stressors on the tropical lakes of Africa have been informed by long-term analyses, but these studies have largely focused on the massive Great Rift Valley lakes. Our objective was to evaluate how recent climate change has altered the functioning and services of smaller tropical lakes, which are far more abundant on the landscape. Based on a paired analysis of 20 years of high-resolution water column data and a paleolimnological record from a small crater lake in western Uganda, we present evidence that even a modest warming of the air (∼0.9°C increase over 20 years) and changes in the timing and intensity of rainfall can have significant consequences on the dynamics of this common tropical lake type. For example, we observed a significant nonlinear increase (R(2) adj  = 0.23, e.d.f. = 7, p<0.0001) in thermal stability over the past 20 years. This resulted in the expansion of anoxic waters and consequent deterioration of fish habitat and appears to have abated primary production; processes that may impair ecosystem services for a vulnerable human population. This study on a system representative of small tropical crater lakes highlights the far-reaching effects of global climatic change on tropical waters. Increased research efforts into tropical aquatic ecosystem health and the development of sound management practices are necessary in order to strengthen adaptive capabilities in tropical regions

Water column data in Lake Nkuruba 1992–2012.

<p>Arrows indicate the timing of observed mixing events. a) thermal stability of the water column at the time of sampling expressed using the Schmidt Stability Index computed following <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086561#pone.0086561-Taranu1" target="_blank">[27]</a>; line indicates trend; b) surface (black circles) and bottom (open diamonds) water temperatures; c) Secchi (black circles) and photic zone (open diamonds) depths (photic zone = Secchi×2.7 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086561#pone.0086561-Margalef1" target="_blank">[28]</a>); d) depth to anoxia determined as the depth at which dissolved oxygen measurements drop below 1 mg L⁻¹; e) thermocline depth determined as the depth where the greatest inflection in the temperature curve occurs over a thickness of 1 m; f) annual occurrences of atelomixis events identified by thermocline depths greater than 15 m (black bar) or situated between 15 and 10 m (grey bar). For a–c) lines and grey bands indicate the nonlinear (GAM) response curves and the 95% confidence intervals, respectively. For d–e) lines indicate a 10-point running mean.</p

Map showing location of the study lake among other crater lakes of the Kabarole, Uganda region.

<p>The X marks the coring site and contour lines represent bathymetry (modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086561#pone.0086561-Chapman2" target="_blank">[15]</a>).</p

Water column dynamics in Lake Nkuruba 1992–2010.

<p>a) annual means of depth to anoxia (squares) and transparency (diamonds), and inferred primary production (circles) expressed as sedimentary concentrations of β-carotene. b) 1992–2007 variations in the concentrations of four sedimentary pigments associated with diatoms (diamonds), cryptophytes (triangles), chlorophytes (squares *concentration values for sedimentary lutein were an order of magnitude higher than the others, and were divided by 10 to fit the graph) and cyanobacteria (circles).</p

Sedimentary pigment variations (1910–2008) and core chronology.

<p>a) Historical variations in selected pigments and important 20^th century human interventions in the Lake Nkuruba catchment. The dotted horizontal line indicates that the uppermost data points (shown as empty circles) were not considered in our analyses due to differences in digenesis between these and older samples <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086561#pone.0086561-Guillizzoni1" target="_blank">[29]</a>. b) Unsupported ²¹⁰Pb concentrations in 16 samples and constant rate of supply (CRS) model used to determine downcore ages (including error bars).</p

Air (line) and surface water temperature (bars) anomalies for Lake Nkuruba over 20 years.

<p>Anomaly computed as the difference from the 1992–2011 mean air and surface water temperatures.</p

Sediment accumulation rate, minerogenic content, recent extreme rainfall events.

<p>a) Sediment accumulation rate (dashed line) in the Nkuruba core (in gr/cm⁻²/year; rates are comparable to other crater lakes from the region, Saulnier-Talbot unpublished) and minerogenic content (the unburnt fraction of the sediment after LOI) of the sediment; number of extreme rainfall events per year and total annual precipitation over the sampling period (data modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086561#pone.0086561-Hartter1" target="_blank">[8]</a>).</p

Observed mixing events in Lake Nkuruba 1992–2012.

<p>Mixing events are defined as sampling days with either <0.2°C amplitude in water column and/or ≤1 mg L⁻¹ DO at surface. All temperatures are in °C.</p