Hydrobiologia 196: 39-50, 1990.© 1990 Kluwer Academic Publishers. Printed in Belgium. 39

The role of aquatic moss on community composition and drift offish-food organisms

Merlyn A. Brusven, William R. Meehan 2 & Russell C. Biggam'1 Division of Entomology, University of Idaho, Moscow, ID 83843, USA; 2 U.S. Forest Service, ForestrySciences Laboratory, Juneau, AK 99802, USA

Received 28 September 1988; in revised form 6 February 1989; accepted 20 March 1989

Key words: insects, moss, diversity, drift, substrate, cover, fish, community, invertebrates

Abstract

Macroinvertebrate density, biomass and drift were studied from moss-covered and moss-free channelsin the South Fork Salmon River, Idaho. Insect densities were compared for 10 different substrate typesand locations involving moss (Fontinalis neo-mexicana), sand, pebbles and cobbles. An ANOVA testdemonstrated that insect densities varied significantly with substrate type (P < 0.05), and that total insectdensity in moss clumps differed significantly from densities in mineral substrates. Insect densities were4-18 times greater in moss clumps than in mineral substrates under and adjacent to moss; sands undermoss supported the lowest densities. During most tests, densities in pebble and cobble substrates adjacentto moss clumps were not significantly different from those found in similar substrates in the moss-freechannel. The 20% moss-covered channel had 1.6 to 7.2 greater insect density and 1.4 to 6.1 greaterbiomass than did the moss-free channel for the tests conducted. Generally, midges (Chironomidae) madeup over 50% of the insect community; annelids were the principal non-insect invertebrates.

In spite of greater insect density and biomass in a moss-covered than in the moss-free channel, we didnot demonstrate universally increased drift of the immature stages from the moss-covered channel, at leastduring daylight hours. As a consequence, we infer that salmonid fishes, feeding primarily on driftinginsects during the daytime, may not derive increased caloric benefit from moss habitats until the insectsemerge as adults.

Introduction

The substratum in streams is home for inverte-brates during much of their lives. The nature of thesubstrate tends to influence the microenviron-ment in, on, or under which invertebrates live. Themajority of earlier studies focused on the role ofmineral substrate and its effects on distributionand abundance of stream insects as reported inmajor reviews by Cummins (1966), Hynes (1970)and Minshall (1984). By contrast, the role of

aquatic macrophytes has been little studied as asubstrate for colonization, food for aquaticmacroinvertebrates, and a habitat that sometimesmodifies the morphological characteristics ofstreams. Studies by Percival & Whitehead (1929),Hynes (1961), Minckley (1963), Egglishaw (1969)and Rabe & Gibson (1984) reported greater den-sities of invertebrates on vascular plants than onbarren substrates.

Aquatic mosses have been studied much lessthan aquatic vascular plants with regard to

40

aquatic invertebrates. Frost & Went (1940), Frost(1942), Minckley (1963), Lillehammer (1966) andMaurer & Brusven (1983) reported appreciablygreater invertebrate densities from moss thanfrom mineral substrates in streams. Increasedsurface area of plant leaves, branches and fila-ments has been given as one of the reasons forgreater invertebrate density in aquatic macro-phytes (Minshall, 1984). Maurer & Brusven(1983) proposed that mosses also filter particulateorganic matter from the water, thereby enhancingan invertebrate community composed primarilyof collectors.

The aquatic moss, Fontinalis neo-mexicana, isoften an abundant and prolific macrophyte in theupper reaches of Idaho batholith streams. Manyof these streams have been impacted by sedimentsbecause of logging, grazing, mining and road con-struction. Stream habitat degradation in this areahas seriously affected fish and fish-food organ-isms (Bjornn et al., 1977). Streams in the SalmonRiver drainage of Idaho are important spawningand rearing streams for chinook salmon(Oncorhynchus tshawytscha) and steelhead trout(Salmo gairdneri).

The uncertain role of aquatic moss in the pro-duction of fish-food organisms, and in modifyingthe physical environment under and adjacent tomoss, contributed to the conducting of this study.The objectives of this study were to: evaluateinsect-moss relationships with regard to standingcrop, density and diversity, and to relate thesemeasures to substrates under and adjacent tomoss; evaluate invertebrate drift from moss-covered and moss-free channels; and makeinferences regarding the role of moss in the pro-duction of fish-food organisms and fish so that amore critical assessment of stream habitat can bemade in future management of streams.

Materials and methods

Study location, channel configuration and operation

This study was conducted in an abandonedspawning channel in the South Fork Salmon

River (SFSR) in central Idaho, USA (44 30' N,1161 00' W) formerly used for spawning and rear-ing juvenile chinook salmon. The channel wasformed by cutting a trench across an oxbow in theSFSR more than 25 years ago. The banks havebecome stabilized by natural herbaceous vege-tation during the intervening years. The channel is160 m long by 3.0 m wide and drops 0.58 m overits length. The upper 27 m of the channel wereused in this study. Flow into the channel from theSFSR was regulated with a steel headgate. Theupper 6 m of the channel were used to mix andequilibrate the flow and insect drift across thechannel by positioning deflector rocks below theheadgate.

The experimental portion of the channel was21 m long. Wooden sills were placed in the upper,middle and lower sections to create laminar flowand to delineate test reaches. The original channelwas divided longitudinally into two, 21 x 1.5-mchannels by positioning partially-imbedded0.28 m-high boards along the sides and down thecenter of the original channel.

Tests were conducted in July and again inAugust in 1980 and 1981. One month prior to thefirst test, surface silt and fine sands were flushedfrom the channels by increasing dischargethrough the headgate. The upper 10.5-m sectionsof the channels were manually covered with smallcobbles (63-80 mm) and the lower sections withpebbles (15-32 mm). The resultant streambedreflected an optimum condition typical of a cleanproductive mountain stream. Pebble and cobblesections remained relatively silt-free and un-imbedded during the summer months of the study.Subsurface sediments were composed primarilyof sand and gravel laid down during formation ofthe river oxbow.

The treatment channel (with moss) was ran-domly selected prior to the July test; during theAugust test, the alternate channel was used as thetest channel. One month prior to invertebratesampling, 35-42 moss (Fontinalis) clumps andtheir attachment substrates (rocks 10-15 cm),representing 20% of the test channel's surfacearea, were collected from the SFSR near thechannel and systematically placed in the test

41

channel. Clumps were selected on the basis ofsimilar size and 'loft', i.e., non-compressed height(ca. 11.5 cm). Small adjustments in clump densitywere made to standardize percent cover betweenmonths because of inherent variations in individ-ual clump sizes. The 20% moss cover used in ourexperiments approximated that found naturally inthe SFSR (Maurer & Brusven, 1983).

To simulate sand deposition under moss,which is common for many of the moss clumps inthe SFSR, we placed 0.91 inm of sand(0.5-1.5 mm) under three randomly-chosen mossclumps in each of the cobble and pebble sectionsof the test channel at the time moss was intro-duced. We allowed one month for the channels toequilibrate to the flows and to be colonized bydrift. In an earlier study, Maurer & Brusven(1983) reported that invertebrates colonized mossclumps to carrying capacity within one week inthe SFSR.

Discharge into the channels was maintained atca. 0.05 m 3/s. Slight variations occurred duringthe colonization period because of changing headpressure of the SFSR. Water depths of17.5-20.0 cm were maintained by adjusting sillheight at the down stream end of the channel, thusassuring complete moss submersion during thecolonization period.

All of the aforementioned conditions wererepeated when we alternated control and testchannels during the August test of each year.

Macroinvertebrate density and biomass

Macroinvertebrate density and biomass weredetermined in, under and adjacent to Fontinalismoss clumps in cobble and pebble sections of thetest channel and in moss-free sections of the con-trol channel on four sample dates. Invertebrateswere sampled with a 0.093-m2 Hess bottomsampler from mineral substrates. For moss, theclumps were measured for length, width and non-compressed height, removed by placing them intoa nylon net (mesh size 0.24 mm) and thoroughlyrinsing them in three water baths. Washings werestrained through a 0.24-mm-mesh nylon net and

the residues preserved in 70 % ethanol. Mats werethen visually examined for remaining inverte-brates which were hand picked and added to thesamples. Because perimeter area of the mossclumps was often greater than the area of the Hessbottom sampler, density was adjusted to theequivalent area of the bottom sampler so thatdensity comparisons in moss could be made withdensities under and adjacent to moss.

For each sample date, three moss clumps wererandomly selected and sampled from pebble andcobble sections of the test channel. Similarly,three 0.093-m samples were taken with a Hesssampler from three mineral substrates under andadjacent to moss clumps from cobble and pebblesection of the test channel (sand under moss -pebble section, sand under moss - cobble section,pebbles under moss, cobbles under moss, pebblesadjacent to moss, and cobbles adjacent to moss).In the control channel, we took three randombottom samples from pebble and cobble sections.The samples were preserved in 70% ethanol. Allsamples were sorted and invertebrates wereidentified to species or morphospecies in the labo-ratory; midges (Chironomidae) were identifiedonly to family. The functional status of the insectswas based upon the classification of Merritt &Cummins (1984).

Invertebrate samples were oven dried andweighed to determine biomass as dry weight(non-ashed) and expressed as g/m'. Samplesanalyzed for biomass were initially preserved in70% ethanol.

Invertebrate drift

We sampled drift of the immature insect stageswith 1.13 x 0.3-m drift nets (0.24 mm pore size).Nets spanned the entire width of the channels sothat total drift production from the channels wasmeasured at selected time intervals.

During July 1980. we conducted a preliminarytest to determine the equitability of drift enteringtest and control channels through the headgate.We took 1-h drift samples at the top and bottomof the channels at 1200 h and 2400 h. Discharge

42

was measured through the nets; drift was enumer-ated as drift density, i.e., number of driftinginsects per m3. This was an important testbecause it provided a measure of assurance thatupstream rock deflectors and flow had beenproperly set at the headgate. Having establishedequitability of drift into the channels, we assumedthat differences in drift leaving the two channelsduring subsequent tests were due to treatmentdifferences rather than to differences in driftrecruitment.

Diel drift was measured by taking 30-min driftsamples, four times daily, corresponding to noon,dusk, midnight and dawn periods during July andAugust 1980 and 1981. Samples were preservedin 70 O ethanol, sorted and identified in the labo-ratory.

During the 1981 tests, we took additional 24-hdrift samples following the diel drift-samplingsequence. Rather than diel sampling, we sampleddrift continuously during daylight and nighttimein order to average out diel drift pulses ofteninherent during these two major time periods ofthe day. The sustained daytime drift is an espe-cially important parameter when related to tissueelaboration at the next higher trophic level, espe-cially to sight-feeding salmonids. To preventclogging, the nets were swabbed periodically, thusconcentrating the drift material into the net apex.Drift material was preserved as previouslydescribed, returned to the laboratory, subsampledand enumerated as invertebrate drift density.

Table 1. Comparison of total insect density, Chironomidae density and insect density without Chironomidae for 10 substratevariables during July and August 1980 and 1981 using the Least Significant Difference (LSD) multiple comparison test. Insectdensities not differing significantly (P > 0.05) for the substrate variables are connected by a common line.

Sample period Insect group Substrate variables

CF PF CU PU CA CC PA PC CS PSJuly 1980 Total insects

(N = 3) Chironomidae onlyInsects w/o Chironomidae

CF PF PA PC PU CC CA CU PS CSAugust 1980 Total insects

(N = 3) Chironomidae onlyCF PF PA CC PC CA PU CU PS CS

Insects w/o Chironomidae

PF CF CU CC CA PU PC CS PA PSJuly & August 1981 Total insects

(N = 6)

Chironomidae only

PF CF CA PU PA PC CC CU CS PSInsects w/o Chironomidae

PF = Fontinalis on pebbleCF = Fontinalis on cobbleCU = Cobble under FontinalisCC = Cobble in control reachCA = Cobble adjacent Fontinalis

PA = Pebble adjacent FontinalisPU = Pebble under FontinalisPC = Pebble in control reachCS = Sand under Fontinalis in cobble reachPS = Sand under Fontinalis in pebble reach

43

Analysis

Insect data were analyzed using a two-way analy-sis of variance (ANOVA) with month and treat-ment (substrate) as test variables. If a significantdifference in density was found due to substratetype, Fisher's Least Significant Difference (LSD)multiple comparison test was used to determinewhich substrate types differed. Because of thepreponderance of midges in the insect com-munity, insect density vs. substrate type was ana-lyzed separately in three ways: 1) total insect den-sity, 2) insect density without midges, and3) midge density alone.

Results

Invertebrate density-biomass vs. substrate type

Data analysis for 1980 indicated that there was aninteraction effect between month and substratetype (P < 0.05); therefore, we analyzed eachmonth separately using a one-way ANOVA. Theresultant analysis showed that all insect-densitycombinations were significantly different due tosubstrate type for each month during 1980.Because a similar interaction between month andsubstrate type was not found in 1981 (P < 0.05)for the three insect-density measures, the sub-strate variables were compared without regard tomonth for that year. ANOVA demonstrated thatall three insect densities varied significantly due tosubstrate type. For both 1980 and 1981, LSD

5(

41c

l 31

21I,C

ii

1180 920 1032 984

omidaeInsects

F S U AC F SU A C F S U AC F S U ACJUL AUG JUL AUG

PEBBLE COBBLEFig. 1. Mean number of insects per 0.093 m2 in five substrate types in pebble and cobble sections, SFSR channel, 1981-1982.F = moss, S = Sand under moss, U = Pebbles or cobbles under moss, A = pebbles or cobbles adjacent to moss, C = control

channel with pebbles or cobbles. n = 3/date/channel section.

44

showed where group differences existed(Table 1). Significant differences between thethree density measures and 10 substrate variableswere generally similar for a given test; departureswere noted, however, for insect densities withoutmidges during August 1980, and July-August1981.

During all experiments, total insect density inFontinalis moss differed significantly (P < 0.05)from densities in the mineral substrates under andadjacent to moss, and in the control channel,irrespective of whether the substrate was sand,pebbles or cobbles (Table 1). During all testsexcept July 1980, significant differences in insectdensity between Fontinalis on pebbles andFontinalis on cobbles were noted, while significantdifferences between mineral substrates wereneither consistently shown nor expected.

Midges were the dominant insects from all sub-strate types, representing 69-88% of the insectcommunity (Fig. 1). Because of their abundance,

500'

400'

eg 300-Co

cm

@ 200-

-100-

N

ChironomidaeOther Insects

I

T C T T C T C T CJUL AUG JUL AUG

1980 1981Fig. 2. Insect density per 0.093 m2 in test (T) and control (C)channels, SFSR. n = 3/substrate type/month. Mean densitycomputed on basis of proportional density of each substrate

type to total area of channel.

100'

80-

I-Z

U.-

C6

60-

40-

20-

Au,

D

E

D

p

E

D

p

E

D

l

FSUAF S U APEBBLE

D

p

E

D

p

E

D

-E

D

E

F S U ACOBBLE

Fig. 3. Percent ordinal insect composition from four sub-strate types in pebble and cobble sections in a test channel,SFSR. n = 12/section. C = Coleoptera, D = Diptera,E = Ephemeroptera, P = Plecoptera, T = Trichoptera,O = other orders. Substrate types: F = moss, S = sandunder moss, U = pebbles or cobbles under moss, A = peb-

bles or cobbles adjacent to moss.

midges were analyzed separately and in combi-nation with other members of the insect com-munity. Insect densities were generally 4-18 timesgreater in moss than in mineral substrates underand adjacent to moss clumps. Sand under mosssupported the lowest insect densities among the10 substrate types tested, representing only 1-6%of the densities found in their associated mossclumps. While in most cases we did not achievea significant difference in insect density betweensand under moss and other mineral substratetypes (P < 0.05; Table 1), the difference in meandensity over the two years was substantial. Sandunder moss supported only 12-27% of the den-sities recorded for pebbles and cobbles undermoss for the four tests.

Cobble and pebble sections of the control chan-nel did not differ significantly (P < 0.05) in totalinsect density and insect density without midges

. . -......a i -.--.a-......t........a I

, I m-1 I I

I LV

I

I IS |

45

rtebrates

I L I X

JUL AUGI X I X

JUL AUGlMb5U 1981

Fig. 4. Mean invertebrate biomass (g 0.093 m- 2) in (T) testand (C) control channels, SFSR. n = 24 in test and 6 incontrol channel/date. Mean channel biomass computed onbasis of proportional biomass of each substrate type to total

area of channel.

from the equivalent moss-free substrates in thetest channel (Table 1; Fig. 1). Only midge densitywas significantly different (P < 0.05) for thischannel comparison during 1981.

The insect community was dominated bydipterans, especially midges (Figs. 2, 3). Dipterandensities ranged from 45-95 % of the total insectcommunity during the four test dates. Midgescomprised over 75% of the dipterans; Simuliumsp. (blackflies) was also a prevalent dipteran.Mayflies (Ephemeroptera) were the second mostabundant insect order followed by stoneflies(Plecoptera) and caddisflies (Trichoptera). Dur-ing summer months, the mayflies Ephemerellainfrequens, Ameletus cooki and Baetis hageni werethe principal species collected. Ephemerella infre-quens attained its highest density in the moss, butwas sparse in the mineral substrate. By contrast,A. cooki occurred primarily in the mineral sub-strate. Combined densities of Plecoptera andTrichoptera usually made up < 10% of the totalinsect density from the various substrate types.Stoneflies, Cultus sp. and Alloperla sp., and the

caddisfly Micrasema sp. were dominant membersin their respective orders.

The addition of a 20% moss cover increasedthe overall mean insect density in the test channelcompared with the control channel (Fig. 2). Meandensity in the test channel was 2.5 to 7.2 timesgreater than in the control channel during the Julytests and 1.6 to 1.8 times greater during theAugust tests for 1980 and 1981, respectively.

With the exception of July 1981, mean inverte-brate biomass showed similar trends, however,the biomass differential between the two channelswas much greater in 1980 than in 1981 (Fig. 4).For all tests, insect biomass was 1.4 to 6.1 timesgreater in the test than the control channel.Insects represented 36-64% of the total inverte-brate biomass for the four test dates; annelidsmade up most of the non-insect biomass.

A highly significant difference (P < 0.0001)existed between substrate types for species rich-ness in both pebble and cobble reaches of the testchannel. Mean species richness was greatest inmoss and lowest in sand under moss (Fig. 5).

20-

E 15-

.! 10

C5 5I'-

+

4-

-FI

F S U APEBBLE

+

-I

+1I

F S U ACOBBLE

Fig. 5. Mean number of species per 0.093 m2 from four sub-strate types in pebble and cobble reaches in SFSR channel,1980-1981. Chironomidae counted as a single taxon. Verticallines show range of variability among samples. n = 12/testsection. Species richness differed significantly between sub-strate types for both reaches (P < 0.0001). F = moss,S = sand under moss, U = pebbles or cobbles under moss,

A = pebbles or cobbles adjacent to moss.

E °%5

o

o'

I .

.I

lA.no

a

·f J

46

Pebble and cobble substrates under moss sup-ported significantly fewer species than similarsubstrates adjacent to moss clumps.

Functional guilds were similar among the sub-strate types both within and between pebble andcobble reaches (Fig. 6). Collectors generallyrepresented more than 80% of the insect com-munity while engulfers comprised 12-20%; in allcases shredders and scrapers made up < 10% ofthe insect community.

Invertebrate drift

A preliminary test conducted during July 1980 toevaluate equitability of drift into the test and con-trol channels indicated recruitment was compar-able (Fig. 7). We recorded only 2.5 and 5.4%greater drift into the test than into the controlchannel during the 1200 and 2400 h sampleperiods, respectively. We viewed this small var-

100-

80-

I-...b"a

.

h&

60-

40-

20-

n,

C

E

C C

E

cC

E

C

E

M

C

E

am

C

E

a

F S U A F S U APEBBLE COBBLE

Fig. 6. Percent functional guild composition from four sub-strates in pebble and cobble sections, SFSR, 1980-1981.n = 12/section. F = moss, S = sand under moss, U = peb-bles or cobbles under moss, A = pebbles or cobbles adjacentto moss. Functional guilds: C = collectors, E = engulfers,

O = other guilds.

10.0-

T-

ca

.60=

eo.M

C;M

5.0'

1.0-

.5-

m VI0 10

Noon Midnight

Test

10 10Noon Midnight

ControlFig. 7. Insect drift (no. m- 3) into (I) and out (0) of test (T)

and control (C) channels at 1200 and 2400 h, July 1980.

iance as acceptable within the constraints of thisfield experiment. We also noted a net increase indrift out of the control channel during these sametime periods, while the test channel was inapparent equilibrium. Insect drift, taken fourtimes daily (noon, dusk, midnight, dawn), reflect-ed a typical diel cycle with highest drift densitiesoccurring at midnight (Fig. 8). Insect drift duringJuly and August 1981 (especially at 2400 h) wasless than during the corresponding months of1980. During most times, over 50% of the driftinginsects were dipterans, especially Chironomidaeand Simuliidae. Mayflies were the second mostabundant order of drift insects, especially at mid-night, and were represented primarily by Baetishageni, B. tricaudatus, B. bicaudatus, and Cen-troptilum sp. Stoneflies and caddisflies were mini-mally recorded in drift. Stoneflies showed a smallnighttime pulse, but represented less than 10% ofthe midnight drift; Cultus sp., Alloperla sp. andCalineuria sp. were the principal plecopteran driftspecies.

To better assess drift production of immatureinsects from the test and control channels, day-time and nighttime samples were taken separately

I I I I I ---- - - .-. --.I I1

_ .s �I M~~ gms II

I

r

20-

15-

10-

5-

A-

15-

10-

5-

a-

4728.4 1980

F[3

I1581

CkireemidlaeSimliiaeOther lstects

- _

T C TC1200 2100

JUL 15

Insect drift (no. m- 3) taken four times daily at 30-min intervals, SFSR, 1980-1981.

in 1981 to reveal more average values for thesetwo major time periods (Fig. 9). Test and controlchannels showed similar overall drift trends; driftduring July was greater than during August, andwas 2.3 to 4.4 times greater during the nighttimethan during daytime. During July, nighttime anddaytime drift leaving the test and control channelswere comparable for their respective time periods.In August, however, drift was 44% greater fromthe test channel than from the moss-free channel,but only during the night.

'U

B.-!:R

U,==

C T C TJUL AUG

Fig. 9. Nocturnal and diurnal insect drift density (no. m -3)of the immature stages for control (C) and test (T) channels,

SFSR, July and August 1981.

B.-

idOr

Fig. 8.

_ _ I l -v

i

-

'z 6�""~idA~~~|

48

Discussion

The aquatic moss, Fontinalis neo-mexicana, is anendemic and often abundant macrophyte in theupper reaches of the SFSR and its tributaries. Anexperimental channel with 20% moss cover sup-ported greater insect density and biomass than amoss-free channel.

Moss appears in different growth forms anddensities in the river. The most characteristic formis that which was simulated in the experimentalchannel where the moss appears as non-contiguous, moderately-spaced clumps with long,free-floating filamentous tails. A second form isrepresented in the river by dense mats composedof many closely-spaced clumps. This form contri-butes to a homogeneous expanse that may ap-preciably alter the hydraulic properties of a river(Edwards & Brooker, 1984). A third form occursin shallow, fast-flowing riffles; in this case, themoss filaments are shorter and appear 'sheared'from the forces of the water and bedload, andfrom greater instability of the substrate. A fourthgrowth form sometimes occurs in backwaterpools and eddies. In these habitats, the moss ismore lentic than lotic and grows more verticallythan horizontally.

In the upper reaches of the SFSR, we observedmoss clumps and mats influencing the hydrauliccharacter of the river and microenvironments inand around them. Sediments and particulate or-ganic matter often collect in abundance in thevicinity of the moss aggregations because the en-ergy and transport capacity of the stream isreduced as the water flows through, over andaround these macrophytic intrusions into thewater column. Because the SFSR transports con-siderable amounts of granitic sands as bedloadannually, many moss clumps and mats (aggre-gation of clumps) cause deposition of sandswithin, and to some degree, downstream from themats.

Sand substrate (0.5-1.5 mm) has been shownto be highly undesirable for most stream insects,except some dipterans and a few specially-adapted species in other insect orders (Brusven &Prather, 1974; Luedtke & Brusven, 1976; and

Bjornn etal., 1977). If indeed, sand was ob-jectionable to most lotic insects in the SFSR, andmoss was a preferred habitat as suggested byPercival & Whitehead (1929), Frost & Went(1940), Frost (1942), Hynes (1961), Lillehammer(1966), Egglishaw (1969) and Maurer & Brusven(1983), we hypothesized that moss would offsetthe negative attributes of its own-createdmicroenvironment of sand beneath the clumps.Our hypothesis was upheld in that a 20% mosscovered channel supported a greater density andbiomass of insects and other invertebrates than amoss-free channel.

The critical nature of sand with respect toaquatic macrophytes in lotic systems may be ad-dressed in the context of ecological patch. 'Patch'in this sense has both a horizontal as well as avertical dimension. Moss overlaying pebble andcobble substrates in natural rivers contributes tosand deposition within the moss' hydraulic pe-rimeter of influence. When moss clumps aggregat-ed into dense mats, serious impaction of the un-derlying mineral substrate by sand was observed.When the moss clumps were widely spaced, e.g.> 1.5 m apart, which is typical for the riffles in thevicinity of the SFSR oxbow, silt impaction of theadjacent mineral substrate was seldom observedas a problem.

Resultant patches of moss and associatedmineral substrates beneath and adjacent to themoss, supported different insect densities, ordinalcompositions and species richness. While we didnot detect significant differences in insect den-sities between sands beneath moss clumps andthe other mineral substrates, the sand habitatunder moss clumps supported lower overall insectdensities than pebble or cobble substratesadjacent to or under moss.

Much can be said about the POM filteringproperties of moss in running waters, and howPOM influences invertebrate density and diversi-ty (Glime & Clemons, 1972; Maurer & Brusven,1983). One may also attribute larger overall insectdensities on plants than on mineral substratesbecause of increased surface area of the plants(Minshall, 1984). When comparing actual den-sities of insects on individual rocks with surface-

49

area densities on plants, Rooke (1984) foundgreater densities on stones.

Having established that a 20% moss cover ap-preciably increased the overall mean insect densi-ty in a test channel compared with a moss-freechannel, we hypothesized that insect drift wouldincrease accordingly. Increased drift from aquaticmacrophytes could be of considerable importancein managing a salmonid fishery and evaluatingstream habitat. Sampling drift on a selected dielbasis (four samples per 24 h) did not clearly sub-stantiate our hypothesis, i.e. that more insectsdrifted from the moss-covered channel than themoss-free channel. In fact, the opposite was trueduring a preliminary test in July 1980. In all cases,insect drift was very low during daylight hoursand high during the night. Similar findings havebeen reported by (Maller, 1954; Waters, 1962;Brusven, 1970; Waters, 1972).

Perhaps greater resolution to this questioncame in 1981 when drift samples were taken con-tinuously during daylight and nighttime. DuringJuly there was no appreciable difference in driftdensity between the control and test channels foreither time period. During August, however, driftwas 44% greater from the test channel, but onlyduring nighttime. We can only speculate as to whydrift production from our control and test chan-nels differed between these two summer monthssince the colonization time, moon phase andhydraulic inflow parameters were similar. Wesuggest that differences in species compositionand age structure of the insect community may beimportant factors. During August we noted a pre-ponderance of early-instar larvae and nymphs.While we did not conduct a critical size-classanalysis of all species, we suggest that the early-instar feeding stage of many species common inmoss may have contributed to a disproportionate-ly large number of drifting insects during August.

While several workers have proposed that driftis density dependent for some species (Waltonet al., 1977; Wiley & Kohler, 1981), we do notbelieve that this phenomenon sufficiently and dif-ferentially operated between the two channels toproduce increased drift in the moss-coveredchannel. This notion is supported by the fact that

insect density was greater in July than in August,and did not produce a disproportionately largerincrease in drift from the test channel than fromthe control channel.

Because salmonids are largely sight feeders andobtain much of their food during daylight hours asdrift, our findings would tend to negate the impor-tance of moss as a habitat augmenting fish foodin the form of insect drift of the immature stages(i.e. nymphs, larvae or pupae). A potentially im-portant, but unresolved question in the trophic-dynamic relationship of moss, insects and fish, ishow adult insects seasonally influence tissue ela-boration at the next higher trophic level. Sincemost of the insect species studied are univoltine,emerge from the water as flying adults, mate andreturn to the water to oviposit, the adult stagecould be especially important to insectivorousfishes. Because adult insects have great powers ofdispersal, their contribution to the food chaincould extend well beyond the moss-covered habi-tats that produced them.

The influence of moss on benthic insect pro-duction in the SFSR is substantial when compar-ed with production from mineral substrates ad-jacent to or beneath moss clumps. In nature, mossclumps tend to be patchy and contribute to sub-strate heterogeneity. While our studies were con-ducted during summer months, it is reasonable toexpect that invertebrate production within moss-covered reaches varies seasonally as does theautochthonous production of the moss itself.

The question of how much moss is good or howmuch is bad in lotic systems is one of considerablerelevance to stream ecologists and hydrologists.Catchments are being altered at ever-increasingrates from agriculture, logging, mining, road con-struction etc. These alterations not only affect thephysical characteristics of streams, but the biotaas well. Aquatic mosses are affected in ways thateither promote or reduce endemic assemblages.Increased moss cover in streams may impact im-portant spawning gravels by causing deposition ofsediments in otherwise productive spawningareas. Expansive moss mats may also increasestream braiding and accelerate changes in stream-channel geometry, while at the same time support

50

much higher densities of fish-food organisms.The complete role of the moss Fontinalis neo-

mexicana in the ecology of streams is largely con-jectural at this time. Evidence from this studysuggests, however, that greater emphasis needs tobe placed on the growth form, distribution andabundance of moss when managing salmonidfisheries in streams.

Acknowledgements

This research was supported in part by the USDAForest Service as a cooperative agreement,Supplement INT-80-110 CA. We wish to thankM. Delong and R. Rhew for their assistance in thestatistical analysis of the data, and to C. Nyberg,J. Ward, D. Wade and E. Hornig for technicalassistance in the field and laboratory. We alsoextend our appreciation to Dr. Albert Lilleham-mer, University of Oslo, Norway, for his con-structive suggestions in preparing this manu-script. Published with the approval of the Direc-tor of the Idaho Agricultural Experiment Stationas Research Paper no. 88754.

References

Bjornn, T. C., M. A. Brusven, M. R. Molnau, T. H. Milligan,R. A. Klamt, E. Chacho & C. Schaye, 1977. Transport ofgranitic sediment in streams and its effect on insects andfish. Univ. of Idaho Forest, Wildlife and Range Exp. Sta.Bull. No. 17, 43 pp.

Brusven, M. A., 1970. Drift periodicity and upstream dis-persion of stream insects. J. Ent. Soc. Brit. Columbia 67:48-59.

Brusven, M. A. & K. V. Prather, 1974. Influence of streamsediments on distribution of macrobenthos. J. Ent. Soc.Brit. Columbia 71: 25-32.

Cummins, K. W., 1966. A review of stream ecology withspecial emphasis on organism-substrate relationships.Pymatuning Laboratory of Ecology, Special Publication 4:2-51.

Edwards, R. W. & M. Brooker, 1984. In Whitton, B. A. (ed.),Ecology of European Rivers. Blackwell Scientific Publi-cations, Oxford, London: 51-82.

Egglishaw, H. J., 1969. The distribution of benthic inver-tebrates on substrata in fast-flowing streams. J. Anim.Ecol. 38: 19-33.

Frost, W. E., 1942. River Liffey, Survey IV. The fauna of thesubmerged 'mosses' in an acid and an alkaline water. Proc.R. Ir. Acad. 47B: 293-369.

Frost, W. & A. E. J. Went, 1940. River Liffey Survey III. Thegrowth and food of young salmon. Proc. R. Ir. Acad. 46B:53-80.

Glime, J. M. & R. M. Clemons, 1972. Species diversity ofstream insects on Fontinalis spp. compared to diversity onartificial substrates. Ecology 53: 458-464.

Hynes, H. B. N., 1961. The invertebrate fauna of a Welshmountain stream. Arch. Hydrobiol. 57: 344-388.

Hynes, H. B. N., 1970. The Ecology of Running Waters.University of Toronto Press. Toronto. 555 pp.

Lillehammer, A., 1966. Bottom fauna investigations in aNorwegian river: the influence of ecological factors. NyttMagasin for Zoologi 13: 10-29.

Leudtke, R. L. & M. A. Brusven, 1976. Effects of sand sedi-mentation on colonization of stream insects. J. Fish. Res.Bd Can. 33: 1881-86.

Maurer, M. A. & M. A. Brusven, 1983. Insect abundanceand colonization rate in Fontinalis neo-mexicana(Bryophyta) in an Idaho Batholith stream, USA. Hy-drobiologia 98: 9-15.

Merritt, R. W. & K. W. Cummins, 1984. An Introduction toAquatic Insects of North America. Kendall/Hunt Pub.Co., Dubuque, Iowa. 722 pp.

Minckley, W. L., 1963. The ecology of a spring stream: DoeRun, Meade County, Kentucky. Wildlife Monographs 11:1-124.

Minshall, G. W., 1984. Aquatic insect-substratum relation-ships. In Resh, W. H. & D.M. Rosenberg (eds), TheEcology of Aquatic Insects. Praeger, New York, N.Y.,358-400.

Miller, K., 1954. Investigations on the organic drift in NorthSweden streams. Report Drottingholm Inst. for Fresh-water Res. 35: 133-48.

Percival, E. & H. Whitehead, 1929. A quantitative study ofthe fauna of some types of stream-bed. J. Ecol. 17:282-314.

Rabe, F. W. & F. Gibson, 1984. The effects of macrophyteremoval on the distribution of selected invertebrates in alittoral environment. J. Freshwat. Ecol. 2: 359-371.

Rooke, J. B., 1984. The invertebrate fauna of four macro-phytes in a lotic system. Freshwat. Biol. 14: 507-513.

Walton, O. E., Jr., S. R. Reice & R. W. Andrews, 1977. Theeffects of density, sediment particle size and velocity ondrift of Acroneuria abnormis (Plecoptera). Oikos 28:291-98.

Waters, T. F., 1962. Diurnal periodicity in the drift of streaminvertebrates. Ecology 43: 316-320.

Waters, T. F., 1972. The drift of stream insects. Ann. Rev.Ent. 17: 253-72.

Wiley, M. J. & S. L. Kohler, 1981. An assessment of biologi-cal interactions in an epilithic stream community usingtime-lapse cinematography. Hydrobiologia 78: 183-88.