By Ted Siler
Canoe Densities
The phenomenon of drift, or the downstream movement of benthic macroinvertebrates in the water column, is an important process in the ecology of river and stream ecosystems. Not only does it provide a source of food for drift feeding fish like trout, but it is also important in the life history of the organisms drifting. Aquatic insect larvae use drift as a method of dispersal, escape from predation, and as a way to locate better food sources.
Tom Waters (1965), who did most of the early work on drift, proposed that drift could be broken up into three separate classes; constant drift, behavioral drift, and catastrophic drift. Constant drift is defined as the occasional drift of benthic species that, for one reason or another, lose their hold on the bottom and enter the water column. Behavioral drift includes those organisms that actively enter the water column by swimming or other means and move downstream with the current. And catastrophic drift includes the organisms that are forced into the water column by spates or other physical disturbances of the benthos.
Many studies have been published on the process of drift but only a small percentage of these have explored the catastrophic aspect of drift.
The behavioral aspect of drift includes a diel (a twenty-four hour period) periodicity that is of much interest to drift researchers and trout fisherman as well. Many, but not all aquatic macroinvertebrates, exhibit a low diurnal and high nocturnal drift rate. This elevated drift after sunset may be an increase of ten to four-hundred times the diurnal drift rate. This elevated drift after sunset may be seen in streams and rivers located all over the world (Colwell and Carew 1976; Skinner 1985; Flecker 1992; Brewin and Ormerod 1994).
Many hypotheses have been put forth to explain behavioral drift but there are two main groups of thought about the entry of invertebrates into the water column; passive versus active entry. Those researchers that propose passive entry into the water column hypothesize that macroinvertebrates are more active during the night, and thus enter into the drift as a result of more activity on the tops of stones and other benthic substrates in the stream (Elliot 1968). This increased activity then increases the likelihood of accidentally being swept into the water column.
Researchers that believe in more of an active entry propose that there is some reason that macroinvertebrates wait and enter the water column at night; they actively release from the bottom and propel themselves into the current to be carried downstream.
The most convincing argument for active entry comes from recent research completed in the Andes Mountains (Flecker 1992). Streams with drift feeding fish were found to exhibit a diel periodicity of macroinvertebrate drift while those streams in higher elevations with no fish exhibited aperiodic drift, that is, with no nocturnal-diurnal cycle. These results have been supported by similar research in other fishless streams (Brewin and Ormerod 1994), as well as controlled studies that used fish odor as drift cues (McIntosh and Peckarsky 1996). It is hypothesized that macroinvertebrates drifting at night are less likely to be preyed upon than those drifting during the day, for the most part. The taxa (taxonic groups such as mayflys) that tend to drift during the day or show no periodicity are smaller and less conspicuous, like midges (Chironomidae) and blackflies (Simulidae).
Taxa showing a preference for nocturnal drift are larger and include some late instar mayflies (Ephemeroptera) and stoneflies (Plecoptera).
The research completed on catastrophic drift deals mostly with floods and water level changes due to release of water by hydro-electric dams. A few researchers have noted instances of catastrophic drift from human disturbances of the benthos due to wading (VanHouten 1986; Barnum 1979), but very little research dealing strictly with benthic disturbances causing abnormal drift rates has been completed. The purpose of investigating drift in the Au Sable is to answer two questions: 1) Are macroinvertebrate drift rates on the Au Sable abnormal during the summer because of the volume of canoes and tubers using the river? And 2) if the drift rate is abnormal during the day, does this affect the "normal" high night-time drift rate of benthic individuals? In other words, is catastrophic drift increasing macroinvertebrate mortality by displacement of normally drifting as well as non-drifting taxa?
Two sites were chosen on the Au Sable to conduct catastrophic drift studies. One site, located approximately thirty meters upstream from the Thendara Road access site, was sampled on 12-13 April and 27-28 April 1996 to establish a normal diel periodicity of drift for the Au Sable. This site was also sampled on 25-26 June for control values to compare with 29-30 June 1996 drift values during high usage of the river.
Sampling also took place before and after the canoe race on 27 July 1996 at this site. The second site was located approximately 20 m upstream from the Stephans Bridge public access site and was sampled on the weekend of 20-21 July and 23-24 July 1996 for a midweek control. All drift sampling was conducted for 24 hours to obtain the diel periodicity of drift.
Drift was collected using standard Wildco® (Saginaw, MI) drift nets measuring 30 x 45 cm with a Nitex mesh size of 0.383 mm. Three drift nets were placed in the thalweg and left for ten minutes each hour to determine macroi invertebrate e drift rates. After drift samples were collected all macroinvertebrates were sorted from detritus and identified, counted, dried, weighed, and ashed in a muffle oven at 550°C for twenty-four hours to determine ash free dry mass (AFDM) of organisms collected. The densities and weights of macroinvertebrates collected were compared between times of high and low public usage of the river to determine if increased river traffic increased drift. Also day and night values were compared to determine if high daytime drift rates caused lower drift rates at night.
Once the invertebrates were picked from the samples the remaining detritus was dried, weighed, and ashed to determine detritus AFDM for odd hour samples. Any increase in AFDM of detritus should indicate increased benthic disturbance and resuspension of detritus and course particulate organic matter (CPOM) into the water column. The drift samples collected on even hours were used to run trichomatic chlorophyll analysis, which estimates the amount as well as the kind of algae drifting in the water column.
Increased physical benthic disturbances should also increase algal drift by sloughing off algae from benthic substrates such as cobble and boulders.
Fine particulate organic matter (FPOM) measurements were also taken every hour. FPOM is detritus that is less than 1mm but greater than 0.45um. A grab sample of water was collected from the river and 500 ml of water was filtered through preweighted 0.45um Millipore® filters. The filters were then dried, weighed, and ashed to determine AFDM or FPOM in the water column. These values were then combined with discharge values to determine the amount of FPOM in the water column passing the study site during a time interval. FPOM values were compared between times of high and low river usage to detect disturbance of benthic substrates as well as depositional zones due to wave action.
Temperature (°C), dissolved oxygen (mg/l), conductivity (ohms), turbidity (ntu), pH, and incident and substrate level light intensity values (lux) were also collected during each sampling. Presently, all FPOM values have been examined as well as detritus and chlorophyll values from drift samples already sorted for macroinvertebrates. Drift samples are continuing to be sorted and identification and enumeration of macroinvertebrates to determine drift densities that take place after sorting is completed.
Results from this study will be published in a later issue of this newsletter.
Determining if abnormal drift rates are present is important to the future management of the Au Sable system. If drift rates are found to be abnormal, further investigations need to be conducted to determine if and how this might affect drift feeding fish populations as well as macroinvertebrate mortality. Other aspects of the ecology of the Au Sable also need to be studied. Parameters such as habitat availability, food supply, water chemistry, and quantity, quality, and source of detrital inputs need to be investigated further to rule our or support the many hypotheses put forth to find out why fish production in the Au Sable is in decline.
Very little historical data is available for the river which makes it difficult to detect what might be changing or has changed in the last three to four decades. It is important to start gathering information about the river now to aid future generations in the management of the Au Sable system.
Acknowledgements:
Major funding for this project was provided by The Anglers of the Au Sable. Drift nets were supplied by The Anglers of the Au Sable and Central Michigan University biology department which also supplied Millipore filters and equipment used for collecting water chemistry parameters. Thanks to Michigan State University, Dr. Tom Coon, Kevin Gardner for the use of the Wa Wa Sum research facilities, and Gary Willoughby for the use of his property to conduct drift studies. Also thanks to Archie Martell, Connie Bazner, Matt Dare, Dan Swallow, and Craig Stricker for their help in the long hours of collecting drift samples. Dr. Donna King, Dr. Bob King, and Rusty Gates have also provided information and encouragement in conducting this research.
Literature Cited
Barnum, J.B. 1979. Effects of rotenone on benthic macroinvertebrates in a Michigan stream. Masters Thesis. Central Michigan University, Mt. Pleasant, MI., 124pp.
Brewin, P.A., and S.J. Ormerod. 1994. Macroinvertebrate drift in streams of the Nepalese Himalaya. Freshwater Biology 32:573-83.
Cowell, B.C., and W.C. Carew. 1976. Seasonal and diel periodicity in the drift of aquatic insects in a subtropical Florida stream. Freshwater Biology 6:587-94.
Elliot, J.M. 1968. The daily activity patterns of mayfly nymphs (Ephemeroptera). J. Zool., Lond. 155:201-21.
Flecker, A.S. 1992. Fish predation and the evolution of invertebrate drift periodicity: evidence from neotropical streams. Ecology 73 (2):438-48.
McIntosh, A.R., and B.L. Peckarsky. 1986. Drift-feeding and benthic predators induce diel periodicity in mayfly behavior. Bulletin of the North American Benthological Society13(1):148.
Skinner, W.D. 1985. Night-day drift patterns and the size of larvae of two aquatic insects. Hydrobiologia. 124:283-85.
VanHouten, J.W. 1985. Effects of fisherman wading activity on benthic macroinvertebrate drift in a Michigan trout stream. Masters Thesis. Central Michigan University, Mt. Pleasant, MI., 72pp.
Waters, T.F. 1965. Interpretation of invertebrate drift in streams. Ecology. 46(3):327-34.
RWOL
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