Browsing by Subject "Limnology"

This dissertation investigates the limnology of the southwestern Peruvian Amazon centered on the Madre de Dios department by examining first the geomorphology and then the ecology and biogeochemistry of the region's fluvial systems.

Madre de Dios, Peru is world renowned for its prolific biodiversity and its location within the Andes biodiversity hotspot. It is also a site of study regarding the development of the Fitzcarrald Arch and that feature's geomorphological importance as the drainage center for the headwaters of the Madeira River - the Amazon's largest tributary and as well as its role as a physical divider of genetic evolution in the Amazon. Though each of these has been studied by a variety of prominent researchers, the ability to investigate all the aspects of this unique region is hampered by the lack of a regional geomorphological map. This study aims to fill that gap by using remote sensing techniques on digital elevation models, satellite imagery and soil, geology and geoecological maps already in publication to create a geomorphological map. The resulting map contains ten distinct landform types that exemplify the dominance of fluvial processes in shaping this landscape. The river terraces of the Madre de Dios River are delineated in their entirety as well as the various dissected relief units and previously undefined units. The demarcation of the boundaries of these geomorphic units will provide invaluable assistance to the selection of field sites by future researchers as well as insights into the origin of the high biodiversity indices of this region and aid in planning for biodiversity conservation.

Secondly this study examines 25 tropical floodplain lakes along 300 km of the Manu River within the Manu National Park in the Madre de Dios department. Alternative stable state and regime shifts in shallow lakes typically have been examined in lakes in temperate and boreal regions and within anthropogenically disturbed basins but have rarely been studied in tropical or in undisturbed regions. In contrast this study focuses on a tropical region of virtually no human disturbance and evaluates the effects of hydrological variability on ecosystem structure and dynamics. Using satellite imagery a 23 yr timeline of ecological regime shifts in Amazon oxbow lakes or "cochas" is reconstructed. The study shows that almost 25% of the river's floodplain lakes experience periodic abrupt vegetative changes with an average 3.4% existing in an alternative stable state in any given year. State changes typically occur from a stable phytoplankton-dominated state to a short lived, <3 yr, floating macrophytic state and often occur independent of regional flooding. We theorize that multiple dynamics, both internal and external, drive vegetative regime shifts in the Manu but insufficient data yet exists in this remote region to identify the key processes.

To complete the investigation of tropical limnology the third study compares and contrasts the nutrient-productivity ration of floodplain and non-floodplain lakes globally and regionally. For over 70 years a strong positive relationship between sestonic chlorophyll-a (Chl-a) and total phosphorus (TP) has been established with phosphorus generally viewed as the most limiting factor to productivity. Most of these studies, however, have focused on northern, temperate regions where the lakes are typically postglacial, isolated and fed by small streams. Relatively little work has been done on floodplain lakes which are semi or permanently connected to the river. This study examines the relationship between nutrients and productivity in floodplain lakes globally through an extensive literature synthesis. Values for total phosphorus, total nitrogen and chlorophyll-a were collected for 523 floodplain lakes, represented by 288 data points while 551 data points were collected for 5444 non-floodplain lakes. Analysis revealed that globally, floodplain lakes do not show any significant difference in the total phosphorus/chlorophyll-a relationship from that found in non-floodplain lakes but significant differences are seen between tropical and temperate lakes. We propose that the term `floodplain' lake should serve as purely a geographical descriptor and that it is lacking as an ecological indicator. Instead factors such as precipitation seasonality, hydrological connectivity and regional flooding regimes are better indicators of high or low productivity in floodplain lakes.

Dissolved organic matter (DOM) is present in all natural waters and modulates

aquatic ecosystems by absorbing light and heat, and because it comprises a complex

mixture of organic molecules including amino acids, sugars, fulvic acids, and humic

material. DOM is derived from dissolution of organic matter and can be altered by

both biotic and abiotic processes that may its structure or mineralize it to CO₂.

Urbanization is a widespread agent of landscape change that can alter DOM

regimes by changing the amount and types of organic matter in the catchment and

by changing the way that water moves through the landscape (transporting DOM

from land to stream). This dissertation examines DOM in urban stream networks,

exploring its sources, bioavailability, and broad patterns throughout the continental

United States.

We determined the role of impervious infrastructure as a proximate source of

DOM to stormwater by a) constructing an annual carbon budget for the roof of a

house as a small catchment nested within the 60 h catchment of an urban headwater

stream and b) comparing the estimated fluxes of solutes and stormwater from imper

vious infrastructure in the catchment. We found that roofs convert nearly one-third

of the leaf litter carbon they receive into dissolved organic carbon (DOC), which

leaves through downspouts. On the event scale, we estimated fluxes of DOC and

total dissolved nitrogen from impervious surfaces that generally exceed the fluxes

that leave the catchment in stream stormflow.

When we compared the chemical composition of runoff from impervious surfaces

to stream stormflow, we found them to be distinct, despite the fact that the we

estimated a volume of runoff from impervious surfaces that generally matched the

volume of water flowing through the stream during storms. Our findings suggest

that a water source other than baseflow and impervious runoff contributes to stream

stormflow, and that a considerable proportion of impervious runoff is lost before it

reaches the catchment pour point.

An experimental incubation of potential DOM sources in the urban landscape and

DOM in stormwater showed that urban DOM is highly bioavailable. The composition

of DOM also became more homogeneous over the course of processing.

Finally, we examined continental-scale patterns and long-term trends in riverine

DOC. Unlike the widespread ’browning’ trends observed in far northern aquatic

systems, we did not find evidence for long-term increases in DOC throughout most

of the U.S. Instead, we both decreases and increases in long-term DOC concentrations

that differed among regions and generally seemed to be driven by changes in weltand

cover. We also found evidence for a marginal effect of impervious surfaces that

increases DOC concentrations at high canopy cover, consistent with our observations

that urban infrastructure can contribute considerable DOM loads in storm runoff.

Together, this research shows that urban stormwater infrastructure functions

as the ephemeral headwaters of the urban stream network. In catchments with

significant canopy cover, these ’engineered headwaters’ collect and transform organic

matter between storms and transport DOM during stormflow.

Diel patterns in river solute concentrations can be derived through diurnal behaviors of organisms living in and around the water. Examples involving nitrate include nutrient uptake by autotrophs during the day and denitrification when oxygen levels are lowest during the night. Due to these biological origins, diel patterns provide information about important biogeochemical processes happening in rivers at broader timescales as well. Studies of diel behavior have been limited to intensively studied reach-scale analyses. This dissertation broadens the scale of these analyses in several ways. First, in an experimental manipulation of nutrient limitation, I demonstrate that diel patterns are indicative of nutrient repletion in an aquatic ecosystem. Second, I develop a method for isolating diel solute concentration patterns from timeseries that also include variability at other scales. I apply this method to high-frequency data at 82 sites around the United States, and find that diel oscillations are greatest during the summer months, and that rivers with higher turbidity, mean nitrate concentration and pH are more likely to exhibit oscillations consistent with dissimilatory processes than consistent with autotrophic processes. Lastly, I summarize high-frequency nitrate timeseries from 142 sites and find that diel patterns are frequent and wide spread within this dataset. The combination of this work broadens the scale at which diel patterns can be studied and adds to the process knowledge that can be derived from them.