Off The Beaten Path – The Index Eagle February 1997 by Bob Hubbard
This article is from the Index Eagle, February 1997 and authored by Bob Hubbard. He has given us permission to reprint his articles but PLEASE DO NOT PLAGIARIZE. This article may not be reproduced without the express written permission of IndexWa.org and/or Bob Hubbard.
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I suppose I must have seemed to have gone a little bit overboard last month, what with my big to-do about what was. After all, just a mud puddle generating rafts of floating slime. But there you go: I’m that kind of guy. I’m used to looking at the ecosystem from the standpoint of one or another of its bugs or other critters: which usually has me looking around trying to find the food chains which sustain them. Often I find the food item first, usually a plant, then notice the bugs hidden within it or the chew or browse marks left by its other apprecianados. The filter-feeding aquatic bug community though, has resisted this approach because it is a community whose food travels to it. rather than the other way around. Speculating on this community’s food source is easy, but proving it is more difficult: like deducing where in a cloud a particular snowflake first started to form. Seeing the algae rafts pull loose from the puddle’s bottom and start on their way downstream gave me a feeling like watching those snowflakes form in the cloud and drop towards the earth below. A feeling like watching creation.
Sharp readers will have noticed that my puddle musings may have answered only part of the question I’d had about the source of the algaes in the river’s load of drifting plankton. Some of the algaes were obviously pulling loose from their anchorages on the bottom and joining the flow, but what had seeded them into the water before that? Algaes could obviously migrate downstream and settle into new territory this way, but how do they get up into the headwaters?
One possible way is for the algaes and diatoms to get stuck to the feathers and feet of ospreys. ducks, eagles, ouzels and other aquatic birds, who then fly upstream, transplanting some of them with every new landing further upstream, even into the high lakes. Many species of feather mites associated with aquatic birds make their living by eating diatoms and other algaes which get caught in the feathers: this association suggests that diatoms (et al.) are dependably common in the feather habitat, and thus able to be transported and transferred in significant numbers. With their thick fur, wading or swimming mammals such as deer and bears could as easily act as taxicabs for the algaes and diatoms. Indeed, there may be hundreds of such freeways in our forests going every which way. Each splashdown site and cruising track used by an aquatic bird, each wading or swimming site used by a mammal or other animal becomes the launching point for a veritable flotilla of transplanted algaes and diatoms.
Given the appropriate environmental conditions, both green algaes and diatoms (which are actually golden algaes) can form heavily-fortified ‘resting cells” (called ‘zygotes” and ‘statospores” respectively) which are able to survive periods of excessive heat, cold and dessication. So even if the ‘taxicab” critter is out of the water for a period of days, when it finally splashes into those cold, clear headwaters some viable algaes and diatoms are likely to be washed into the stream there, and given the newer, better conditions. These will germinate into their more normal “vegetative” forms.
During the normal course of things, stream levels go up and down in response to weather events, and stream bottom rocks which are one day covered with water may be high and dry the next. The algae’s and diatoms on the newly-exposed beaches generate a lot of resting cells as they dry out and when the winds blow over the beaches these zygotes and statospores are carried away along with the other beach dust. Winds blowing upriver constantly reseed the entire length of it with an eclectic mix of species and even downriver winds in the Western Cascades might bring loads of resting cells from Eastside streams, where these same winds are blowing upriver.
Back in a stream again, these revitalized algae’s and diatoms will either get stuck to a rock or something in the stream channel, or will be born along in the flow of water as freshwater plankton. Plankton is the Greek word for wanderer, and it is used collectively to describe a diverse array of free-drifting organisms, regardless of whether they are plant or animal in nature.
Receiving their energy from sunlight, the drifting algae’s and diatoms use it to pull inorganic nutrients (such as phosphorus. magnesium. carbon dioxide, etc) out of the water they are bathed in. so that as they drift they grow and reproduce. Nutrient-rich waters can support faster growth, other factors remaining the same, but near the river’s headwaters, its nutrient value is low and thus the algae’s growth rate is slow, except near localized nutrient concentrations. So the underwater storm of photosynthesizers gets off to a pretty slow start, with each scarce ‘flake” taking a long time to grow and split into two flakes.
Some of the diatoms and algae’s have the luck to wash into quiet backwater pools where a lot of decaying leaves and sticks are laying on the bottom. The stored energy of these organic riches is slowly digested out of them by bacteria, who break down the sugars and complex organic molecules. Some of the organic nutrients are leached out by the water, which flows over the sticks and through the leaf packs and comes away somewhat enriched, like a very weak tea. The mites and insects in the waterlogged organic debris may help to make the ‘tea” stronger by their action of fragmenting the leaves and sticks, which yields more surface area to leak both organic and inorganic nutrients out of. Algae’s and diatoms, as noted, make use of the inorganic nutrients, while the bacteria are the major initial ingesters of the organic molecules.
A diverse array of protozoans, from amoebae and paramecia to rotifers and hydras, prey on the bacteria and algae’s and diatoms, and serve in turn as food for a multitude of aquatic mites and insect larvae. Surges in the flow of streamwater wash a certain number of these micro decomposer organisms out of the quiet spots and into the main streamflow, where they join the drifting algae’s and diatoms as members of the planktonic community.
Pieces of leaves and wood fibers, some of them quite large, join the plankton fleet too, sent on their way by chewing insects, whose sharp but clumsy mouthparts impart a certain sloppiness to their mealtakings. And of,course. the bites which didn’t get fumbled also make their way back into the stream ecosystem as microfeces, and these drift along as other plankton particles. The sticky algaes and diatoms often attach to the surfaces of the particles of leaf, fiber and rnicrofeces, and the nutrient enrichment there allows fast and lush growth. Pieces of leaf and detritus drifting for long, typically become heavily encrusted with diatoms and algaes.
This example streams plankton community is getting complicated and diverse pretty fast: now we’ve got a drifting army of pieces, some single-celled and quite small, others much bigger, like the diatom-covered microfeces and leaf flakes. We’ve got single-celled algaes and diatoms in both free-drifting singles and multiple-celled clumps, pieces and strands of multicelled algaes, bacteria, protozoans. copepods, water fleas. nematodes and others. The average distances between the drifting pieces are getting shorter and shorter as more and more pieces join the flow: the river takes on some of the character of a giant freeway sparsely populated with traffic, at least from the standpoint of someone drifting along with them. To someone on a riverbottom cobble, it probably more resembles a snowstorm of debris flying by in the liquid wind.
In certain rare stretches, the bottom of the river is smooth bedrock. The rest of the way the river bottom is made of loose rocks and cobbles, and where the water slows down enough, fine gravels, sands and silts. All of these bottom types except the bedrock are made of loose pieces of rock, which pile up and leave spaces between them. These spaces are very important, for they interconnect and allow water to circulate and flow down in the sub-basements of the stream bottom. Cover the bottom of a sloping trough with gravel and then pour water through the trough and you’ll see the same thing. Because of irregularities in the stream bottom contours, in some places water from the open stream is pushed down into the inter-gravel pore spaces while in other locations it is being pulled out of them. The water’s flow in a stream channel, being turbulent, sooner or later carries every particle of plankton down to near the bottom, where it runs a chance of being pulled into the inter-gravel spaces or brushed across the surface of the stream bottom. These are the habitats of the stream’s filter-feeding insect citizens.
There may be a good many of these citizens, especially in the stream system’s lower reaches where just the algae and diatom part of the plankton storm is so intense the very water is turned a murky green. In the upper reaches of the same stream, where the trees and vegetation shade much of the stream, the plankton storm is sparse and many of its flakes are relatively large because of the preponderance of decaying leaves as a plankton contributor. Here, the filter-feeding community may be sparse too.
On the bottom of the stream, differences in flow velocity cause a variety of bottom types to form where different sized particles drop out of suspension in the water. In pools and swirls, pieces of decaying leaves skitter and bounce around like burger wrappers on a bare lot, forming temporary piles and drifts, then blowing away again. Here and there small piles of leaf pieces and twigs remain steady, and only by looking close do we find that some of them are really just rudely-made tents, not true piles, and inside these tents are the larvae, or young of caddiceflies of the family Hydropsychidae. The tents, or retreats, have no fixed design and are glued together with silk extruded from the caddicefly’s mouthparts, using pieces of locally-found leaves, sticks and rocks. Some of them look like patchwork pup tents while others look like multi-peaked Bedouin tents. But regardless of floorplan, the entrance usually tends to face across the current and a special catch-net is built out in the ‘porch area”. The Hydropsychids hang out in their retreats and wait for plankton particles to catch in their nets.
Many Western streams have 3 or 4, or even 5 species of Hydropsychids inhabiting them with a sort of progression of those species occurring, as the stream goes from headwater brook to tailwater river. The largest species, making the largest mesh nets. tend to be typical of the upper reaches, and these tend to catch the biggest plankton flakes as well as the occasional drifting insect, which is also devoured. Travelling downstream, plankton storm passes through communities of ever-smaller filtering species, who use ever-smaller mesh sizes in their nets, so that finally it encounters meshes so fine that tiny individual diatoms and single-celled algaes are caught and retained. Other caddiceflies. of the family Polycentropodidae, make their retreats exclusively of silk mesh, building them in the form of tapering tubes which lead like miniature silk tornados into the inter-gravel spaces of the stream bottom. These retreats act as their own catch-nets, funneling plankton down to the ‘trumpetnet caddicefly” larvae in their down current ends. Other members of the same family spin fine-filtering sheet nets over hollows or depressions in the rocks and live beneath them, plucking food from the mesh. A third family of filter-feeding caddiceflies. the Philopotamidae, has members who make long silk mesh tubes as their retreats, but they make them as parallel-sided tubes called ‘finger nets”, which are closed at the downstream end, open at the upstream end, and glued by their lower edges to the upper sides of stream bottom rocks, so that the current holds them open like windsocks and gradually fills them with plankton.
Several genera of Chironomid midges also feature larvae which make silk catch-nets for catching plankton: the meshes on these are often small enough to trap small diatoms and large protozoans. Other fine-mesh or fine-sieve filterers include blackfly larvae, discussed in an earlier column [Sept. ‘94), and freshwater sponges and freshwater clams who do their filtering inside their bodies, rather than with external nets. Several families of mayflies feature larvae with fringed forearms or other body parts which catch plankton like combs and hold it for eating.
When you consider that just one little filter-feeder might clear the water of thousands of pieces of algaes, diatoms, leaf scraps, microfeces and other particles in it’s larval lifetime: and that a one-meter stretch of a river channel might harbor hundreds, even thousands of filter-feeders, then you can start to see where their contributions to water clarity start to add up. Of course, most of these filter-feeding critters are insects, such as caddiceflies, and they eventually reach maturity and go flying off to find mates. Along the way, many of those fall prey to hungry fish birds, bats, or small mammals or amphibians. The great variety of filter-feeders ensures that there will usually be some species around in useable pupal or adult form (rather than hidden larval form), thus benefiting the birds and bats of the terrestrial forest community nearly year-round.
Filter-feeders are major recyclers in the forest web. Think of how many of them there are on (in?) a stream bottom, and how much plankton each one catches and transforms into flesh. Think of how that flesh represents a unit of cleaned water and that it makes the rest of the water just a little bit clearer because that unit’s worth of particles are no longer in it. From a trout’s point of view, this is creating food from waste.
If I seem overly obsessed with the unglamorous in nature it’s because the glamorous already seem well-met with suitors and don’t want for attention. The filter feeders and their prey, the freshwater plankton, are hardly as exciting as the antelopes and the grasses of the Serengetti or the caribou and the lichens of the North Slope, but in their own environment they are equally important. Trout, lions and wolves respectively depend on such grazers to provide them with meat to eat, crafted from local plant life. (O.K.. sometimes not so local in the filter-feeders’ case.) In the stream ecosystem this food chain is not so easy to observe: filter-feeders in particular spend most of their lives totally hidden from view on (or in) the stream bottom, and the planktonic community, though exposed to direct view, is normally made of such small members as to be functionally invisible in the moving water to the unaided eye.
In this largely unheard-of world, drifting plants powered by sunlight scavenge the waters of inorganic nutrients, bacteria scavenge the organic nutrients, and hidden insect larvae filter the waters of the drifting algaes, decaying leaves and other organic particles, cleaning the waters as a result and keeping the nutrients on site.
Filter-feeding is an elegant way to turn otherwise lost nutrients (once it’s dissolved or on the way downriver, how else do you get it back before it’s gone?) into local environmental gain: attaining in two steps the form of the ever-useable, edible insect, that universal coin of nature.
Tags: Bob Hubbard, Index Eagle, off the beaten path
the Index Eagle | Ed | 
February 18, 2009 4:00 am | 
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