Nyanza Project Graduate Student’s Report
Peter McIntyre
Cornell University
I had two roles in the 2001 Nyanza Project. First, I was the Biology Teaching Assistant for the course, assisting Dr.
Ellinor Michel in guiding the students through the lecture, laboratory, and field portions of the course. My duties
included organizing the biology lab exercises, delivering a lecture on ecosystem ecology, and assisting with student
research projects.
My second role in the course was as a graduate student researcher. I am a PhD student in the Ecology and
Evolutionary Biology program at Cornell University, and my dissertation research focuses on the importance of
herbivore diversity in tropical freshwater ecosystems. My work is primarily field-based, and the littoral zone in
Lake Tanganyika is one of two systems that I am studying. My first introduction to Tanganyika was as a Nyanza
undergraduate student, and it was that experience that inspired me to continue working in this fascinating system.
With support from the graduate student supplement to the Nyanza REU grant, I pursued two broad research topics in
2001. First, I used a combination of experiments and surveys to address links between herbivores, nutrient cycling,
and primary productivity in the littoral zone. To evaluate the influence of herbivory and nutrient limitation on algal
standing stocks and productivity, I used nutrient diffusing substrates. This involved deploying many clay pots that
were enriched with mineral nitrogen (N), phosphorus (P), both, or neither. In addition to the nutrient treatments,
some pots were protected from herbivores using a plastic mesh cage, while others were left unprotected. After
several weeks of growth, the substrates were harvested, and algal biomass was measured using chlorophyll a and
loss-on-ignition. Both measures indicated suppression of algal growth by N-supplementation, but without any
evidence of differences in heterotroph biomass. This result is difficult to explain, but the same pattern emerged in a
similar experiment in 2000, and this continuity suggests that the pattern reflects a real, yet-to-be-identified feature of
algal growth in Lake Tanganyika. I also measured rates of primary production and respiration on all substrates, and
found that net primary production was significantly higher on substrates supplemented with P (alone or in
combination with N). This suggests that P is more limiting than N for epilithic algae in the littoral zone.
To document ambient nutrient availability in the littoral zone, I collected weekly water samples along transects from
shoreline outward at two littoral sites. Water was collected 1 m below the surface at 5 m intervals from shore to 20
m away, and again at 125 m. All samples were filtered, frozen, and returned to the USA for chemical analysis.
Concentrations were near minimum detection limits for all compounds (NH4 ~ 1-10 ppb; NO3 ~ 0-3 ppb; PO4 ~ 01.5 ppb; total N ~ 70-170 ppb; total P ~ 1.5-4.3 ppb). Several compounds were somewhat more concentrated near
shore, and there was significant variation between weeks in concentrations. The sites were fairly similar, and
temporal patterns appeared to be uniform at both sites. The molar ratio of total N to total P in these samples varied
somewhat, but was always greater than 40:1, indicating likely P-limitation on algal growth. To test this,
phytoplankton samples were collected along with water samples. PO4 concentration showed a strong positive
correlation with phytoplankton chlorophyll, while NO3 has a slight negative correlation with chlorophyll. The
combination of ambient TN:TP and phytoplankton patterns are further indication that, during the summer of 2001, P
limited algal productivity more strongly than N.
In many freshwater ecosystems, consumers may influence nutrient availability by recycling food-derived mineral
nutrients. The high density and biomass of consumers, coupled with low nutrient concentrations, in the littoral zone
of Lake Tanganyika indicates that this pathway could be important there. I measured nutrient excretion rates and
ratios for seven species of fish (Lates, Lamprichthys, Opthalmotilapia, Limnotilapia, Petrochromis, Tropheus,
Simochromis) and one snail (Lavigeria grandis) by placing freshly-caught consumers into clean plastic bags of
water, and measuring changes in nutrient concentrations relative to an empty reference bag. TN:TP in liquid
excretion ranged from 10:1 to 80:1 (molar), and there were significant differences between fish families in excretion
rates and ratios. Snails were also significantly different from all fish. These data indicate that the composition of
the littoral consumer community may influence nutrient concentrations and the balance between N and P limitation,
but further data is needed to make useful extrapolations.
My second research emphasis was using stable isotopes to define trophic relationships among littoral herbivores.
This effort had two components. First, I continued my collections of samples from snail species at many sites. The
Michel group has documented both spatial and temporal turnover in snail assemblages along the eastern coast of
Lake Tanganyika, and the stable isotope data can be used to measure the stability of trophic relationships in time and
space at many of these sites. Second, I collected samples from all of the herbivorous fish and an assortment of fish
at higher trophic levels at one site (Jakobsen’s Beach). These samples will be used to test for trophic partitioning
between the numerous herbivores at this site. Unfortunately, none of the 2001 isotope samples have been analyzed
yet, so I have no results to report here.
The work described above will form a major component of my dissertation, and I plan to return to Lake Tanganyika
to continue these projects in 2002. None of this work would be possible without support from the Nyanza Project in
the form of both financial assistance and access to research equipment (especially SCUBA gear, transportation on
land and water, and laboratory equipment). Along with the dissertation projects described above, I worked closely
with all of the Nyanza biology team members to develop and execute their projects, and all of them contributed their
time and efforts to assisting with my work. Such a collaborative atmosphere is rare in academic science, and the
Nyanza administration and staff are to be commended for fostering it.
I would also like to point out a few of the ways that the Nyanza Project benefits from inclusion of graduate students
such as myself. The presence of graduate students at the site enlarges the pool of experienced researchers that
undergraduates interact with, and takes a bit of the burden off of the faculty. This is important both socially and
academically; the intermediate status of graduate students facilitates our role as liaisons between the students and
teaching staff. For students from teaching colleges, Nyanza is often their first opportunity to observe graduate
students, and this exposure helps to prepare them to evaluate their own options for further study. Even students
from research universities seem to enjoy the opportunity to observe graduate student researchers in action, and often
seek both general and specific advice about graduate school. Finally, Nyanza graduate students expand the scope of
the research that the Project can accomplish, contributing to its success in deepening our understanding of ancient
tropical lakes. For all of these reasons, support for graduate student research should remain a component of future
Nyanza Projects.