Aquatic Biology 2002
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Exam 3
Instructions: This exam is open notes and you may take as long as you like to complete it. Please do
not work with anyone else on the exam. All answers should be typed, and no longer than one page long
(I will not read any more than one page). Any figures you may wish to include must be drawn on the
page with the text. Be sure to indicate which question you are answering, but do not write out the
question. 50 pts total, 16.7 pts each.
Answer 3 of the 4 questions below.
Question #1. Compare and contrast the pelagic food webs of a lake dominated by a zooplanktivorous
fish, and a lake dominated by a planktivorous fish. … (as below)
Question #1, revised. Compare and contrast the pelagic food webs of a lake dominated by a
zooplanktivorous fish, and a lake dominated by a piscivorous fish. Sketch a food web that shows
pathways of energy transfer between trophic levels and use this sketch to guide your discussion of
trophic cascades. Be sure to discuss the relative biomass of the trophic levels in the two lakes, and how
and why size structures of zooplankton and phytoplankton populations might differ between the two
lakes. What are the three conditions necessary for trophic cascades?
In order for trophic cascades to occur (and for us to detect them), 1) consumers (here piscivores,
zooplanktivores or large zooplankton) must be able to control the biomass of their prey, 2) it must be a
simple foodweb (and well known, but in complicated foodwebs, such as ones that contain omnivores,
trophic cascades are unlikely), and 3) consumers (primarily the zooplanktivores and the large
zooplankton) must have size selective predation. In my figure, I’ve sketched two really simple food
webs, one dominated by piscivores (fish-eaters), and the other dominated by zooplanktivores. The size
of the oval suggests the relative biomass of each group in the different lakes. Thus, in the piscivoredominated lake, piscivores are abundant and reduce the abundance of zooplanktivores. As a result, the
abundance of large zooplankton (like Daphnia) is high, and the large zooplankton are able to consume
enough algae to control its population and reduce its relative biomass. In the lake dominated by
zooplanktivores, the abundance of large zooplankton is low due to consumption by size-selective
zooplanktivores. Consequently the zooplankton are unable to control the biomass of phytoplankton,
which are found in higher abundances than in the piscivore-dominated lake. Phytoplankton species are
likely to be either very small in the lake with large zooplankton because large zooplankton consume
the larger phytoplankton or phytoplankton could be dominated byspecies too larger (or undesirable) to
be filter out by zooplankton (i.e., cyanobacteria).
piscivore
zooplanktivore
zooplanktivore
large zooplankton
large zooplankton
phytoplankton
phytoplankton
Question #2. The river continuum concept provides a framework for integrating predictable and
observable biological features of river systems. Explain how macroinvertebrates (in terms of functional
feeding groups) are predicted to vary along the river continuum from headwater stream to large rivers.
Consider environmental conditions, light availability, leaf litter input and the ratio of primary
production to respiration.
The RRC assumes that headwater streams run through forested watersheds. The overarching trees
block light for photosynthesis in the streams, and are a source for leaves that fall into the stream.
Consequently, shredders and collectors are common because these organisms use allocthonous
carbon sources (i.e., terrestrial leaves). Shredders, like amphipods, isopods, some caddisflies) break
up the leaves; collectors, (some caddisflies, mayflies, etc) gather the resulting FPOM and use that as
their carbon source. Primary production (P): Respiration (R) < 1
Mid-order streams receive more light and less leaf litter input because they are wider with fewer tree
arching overhead. Water is still clear, so most carbon originates from benthic algae and rooted
macrophytes (P:R > 1). Grazers dominate the invertebrate community. Collectors are also abundant,
and consume FPOM flowing from upstream.
High-order streams are often wide and slow-moving. Benthic algae is light limited due to high
turbidity such that primary productivity (photosynthesis) is less than respiration (P:R < 1 again).
Collectors are the dominate macroinvertebrates, persisting on alloctonous carbon sources (FPOM
from upstream).
Predatory macroinvertebrates are about 10 % in all area.
For more information in the original theory see Vannote, R. L. et al. 1980. The river continuum
concept. Canadian Journal of Fisheries and Aquatic Sciences 37: 130-137.
Question #3. The runoff of water into rivers and lakes has large impacts on hydrology and water
chemistry. What terrestrial characteristics impact the amount (and quality) of water runoff into streams
and lakes? How does runoff impact the hydrology, water conditions, and biota of aquatic systems (be
specific)? Explain you answers.
Terrestrial characteristics:
Ground cover (interception and evapotranspiration by plants) – less equals more runoff
Slope – greater slope has more runoff
Percent impervious surfaces – impervious surface increases runoff, decreases groundwater
recharge; concurrent loss of vegetation decreases evapotranspiration and interception. Runoff often
carries with it water-borne pollutants that are not filtered out by terrestrial vegetation before reaching
lakes and streams.
Infiltration/permeability of surfaces/soils- the more permeable the soils, the more water will
infiltrate and the less water will run off the surface
Erodibility of soils and nutrient content - influence the chemistry and nutrient content of stream
and lake waters
Hydrology issues:
Reduced groundwater recharge causes a lowering of the water table (and loss of some surface
waters) and loss of aquifers.
More runoff means increased peak discharge in streams, with a shorter lag time between the
rain event and peak discharge, and a quicker subsidence to base flows. These flashier systems are
more prone to sudden flooding.
Water conditions:
Concentrations of nutrients and cations are often higher in watershed that have been logged
(i.e. plants are not present to take up nutrients).
In areas with denuded soils (agricultural lands, clearcuts), runoff can carry suspended solids
that smother gravel beds and other sediments and increase turbidity. In urban areas with storm
sewers, excessive organic matter (mostly leaves and lawn clippings) carried to the lake or stream
during runoff events can cause increased decomposition and associated low concentrations of
dissolved oxygen.
Biotic responses:
In flashy systems, organisms may be washed downstream. Floods are particularly problematic
if they occur during the ‘wrong’ time of the year because they can wash larval or young forms
downstream.
Inputs of excessive nutrients can cause eutrophication; inputs of pollutants can kill aquatic
organisms.
Inputs of sediments smother gravel nests of salmon and other stream-breeding fish. Excessive
organic material reduces oxygen through decomposition, increasing the prevalence of low-oxygentolerant organisms especially in lakes or slow-moving streams.
Others also discussed the effects of logging on runoff quantity and quality.
Question #4. Most of you are studying a specific organism or group of organisms in your independent
group project. Name the organism you are studying and give a brief description of its natural habitat.
Explain the trophic position of your study organism(s). Would it be impacted by a tropic cascade?
How? (If you are not measuring some aspect of an organism, answer this question about on organism
(be specific) that would impact your study.)
I was more likely to take points off for incorrect statements than for lack of information…