Posts Tagged ‘invertebrates’

~Jefferson River Montana~

Friday, June 25th, 2010

The Jefferson River Flooded last weekend. All the hay fields in the area were flooded and roads that have not been submerged in recent history became gentle rivulets. The High water is good for the Jefferson River because it has had many years of below average flows–and fine sediments have built up among the interstitial spaces were many invertebrates that are important fish forage live.

When Snorkeling in recent years, I have found that if you pick up a cobble, there is beneath it only sand. Typically, you might expect to find more cobbles under cobbles. Among the sand grains are the burrows of several sediment dwelling invertebrates (e.g., Hexagenia and Ephoron sp.). According to locals, the river once supported impressive hatches of large stoneflies (Hesperoperla pacifica, Pteronarcys sp. etc).

When sediment fills in the the area under the stones, much of the habitat used buy large stone flies is lost because the interstitial spaces are simply too small for them to pass through. This results in smaller populations (of large invertebrates), and ultimately smaller hatches. It may also reduce the forage available for fish. For example, it appears that sedimentation of the Jefferson River may have caused Burrowing dragonflies (Gomphidae) to replace many of the large predatory stoneflies typically expected. We sampled the Jefferson River several years ago and found about 2-3 gomphid dragonflies per square foot and about 0.3 large stoneflies in the same area–the dragonflies were nearly 10x times as abundant as the stoneflies.  We are hopeful  that high river levels will scour sand from interstitial spaces and improve survival of stoneflies. This could result in an improvement in fishing a few years down the road…

Aside from benefiting river ecology, another effect of the flood is that pools and back waters that have not been flooded for years have now been nicely inundated for a week or more. Mosquito eggs can remain dormant for several years–until they become wet.  We found hundreds of thousands of mature mosquito larvae and pupae among the grassy ditches and fields last weekend. There were so many larvae, that in just a few minutes we observed hundreds of larvae washing across the road in shallow riffles (picture below).  By this weekend or early next week the Jefferson Valley will likely be swarmed by endless squadrons mosquitoes… and it has been such a nice spring.  If you can tolerate the bites and buzzing it might be a nice weekend to try mosquito-mimic flies… and if you have a friend who owns a hay field… maybe see if you can get permission to fish it!

mosquitoes float across the road

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~Holston River Tennessee~

Thursday, June 24th, 2010

The Zone of periodically inundated waters is called the “varial zone” and the community structure can deviate dramatically from the structure of permanently inundated river bottom.

RiverContinuum.org

We are gearing up for our joint venture with the Academy of Natural Sciences to sample the Holston River near Kingsport Tennessee. I last surveyed this river when I managed the Invertebrate Zoology Research section of the Academy’s Patrick Center for Environmental Research in 1997.  The Academy will be collecting water chemistry, algae, fish, qualitative non-insect macroinvertebrates, and quantitative insects (that’s me!) From July 10-July 20 2010.

The project is difficult because the flow of the river fluctuates with hydro-power generation. High water can present safety challenges, but it can also produce sampling challenges. For example, we don’t want to sample part of the river that was bone-dry two hours earlier… it would definitely cause confounding results when the data are compared to locations that were sampled in perennial flowing zones.  The Zone of periodically inundated waters is called the “varial zone” and the community structure can deviate dramatically from the structure of permanently inundated river bottom.  Fortunately, on the Holston we can tell if the rocks were recently exposed to the air for long time periods because stones become crusted with algae or sediment.

Spending a little time identifying the varial zone boudary can really save you from heartache latter, when you try to explain why the study’s results are confounded by sampling issues.  To help avoid these problems, we will spend a day visiting the sites to grab flow measures and identify the amount of lateral/vertical changes in river height (depth).  Also, local news papers usually publish river gauge stage readings and the USGS web page can often provide real-time river height information.  One thing to remember is that these readings are usually reported from dams or bridges, and it may take several hours from from the time a flow spike is recorded and the time it reaches your sample location. If you spend sometime before sampling you can get a feel for time of lowest water at each location.

The Sampling device we will use is the Portable Invertebrate Box Sampler (PIBS)– for those in the know, it is like a rectangular, enclosed Surber Sampler with a foam-sealing base.  It can be used in water depths from 10cm to ~35-40 cm without modification. Therefore, in rivers like the Holston, it is important to try to sample at the period of lowest water so you are sure to collect from the perennial zone… Remember, if the water-level is 50 cm higher than base flow, it will be too deep to reach the perennial zone with the sampler… The Surber would require even a shallower (<30cm) high flow to be effective. Hess samplers are highly variable in their construction and can be modified to work in deeper water, but remember the limiting factor is the length of your arms… if you can’t reach the bottom of the sampler to scrub the cobbles, you cant collect the sample.

I am personally excited by this trip because the downstream site, is one of the most diverse benthic assemblages… it is absolutely lovely!

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