Allochthonous plant litter as a source of organic material in an oligotrophic lake (Llyn Frongoch)

R. D. G. Hanlon The University College of Wales, Aberystwyth, Dyfed, Wales, U.K.

Keywords: plant litter, organic material, production

Abstract The importance of airborne plant litterfall as a source of organic material was evaluated for a small

oligotrophic lake. The airborne plant litter input was estimated to be 100 kg yr’ (45 kg C. yr-t), which represents approximately 5% of the phytoplankton productivity. The quantity of plant litter entering the lake followed an exponential decline with distance from the shore.

Introduction

The energy input into lakes is derived from two main sources; the autochthonous organic material originating from aquatic macrophytes, phytoplank- ton and benthic and epiphytic algae, and the allochthonous material in the form ofdissolved and particulate organic matter from stream flow and airborne terrestrial plant litter. The allochthonous organic matter has been considered an important energy source for the benthic invertebrate popu- lations of lakes (Hall & Hyatt, 1974; Pieczynska, 1972a) and streams (Minshall, 1966; Kaushik & Hynes, 1968; Cummins et al., 1973) and the autumnal litterfall from deciduous trees and shrubs growing on lake and stream margins may provide a substantial proportion of this organic material. In stream ecosystems plant litter may represent 40-99% of the energy entering the system of which some 40-50% consists of autumn shed leaves (Nelson & Scott, 1962; Cumminset a/., 1966; Fisher & Likens, 1973). Deciduous leaf litter in some large eutrophic lakes may represent only a small propor- tion of the total energy input (as little as 0.2-0.3s of the primary production from phytoplankton, Szcze- panski, 1965; Gasith & Hasler, 1976) but it may make a more significant contribution to the organic carbon budgets of small oligotrophic lakes, particu- larly those with highly vegetated shorelines. The

quantity of leaf litter entering a lake will be dependent on the degree and type of afforestation of the lake margins, as well as the topography, climate and edaphic conditions of the catchment area.

The aim of this research is to estimate the input of airborne plant litter into a small oligotrophic lake and determine its importance as a source of organic material.

Study area

Llyn Frongoch (National Grid reference S.N. 721 753) is a small (7.2 ha), shallow (4 m deep) oligotrophic lake situated at an altitude of 300 m in the highland plateau of Mid-Wales. The lake was originally constructed as a reservoir and lies in a shallow basin with a catchment area of 1.15 km*. The lake is surrounded by rough pastureland and has several species of trees and shrubs growing along its margins. Approximately 23% of the 1450 m shoreline is covered by trees, 8% is lined with Beech trees (Fagus silvatia L.) interspersed with several Ash (Fraxinus excelsior L.), Hawthorn (Crataegus monogyna Jacq) and Rowan (Sorbus acicuparia L.) trees and the remaining 15% is lined with the common osier (Salix viminalis L.).

Hydrobiologia 80, 257-261 (1981). 0018-8158/81/0803-0257/$01.00. 8 Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

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Materials and methods

Leaf litter traps were constructed following the suggestions of Phillipson (1971) and consisted of conical nylon mesh bags suspended from metal hoops of area 2000 cm*. The litter traps were attached to wooden stakes which were driven into the lake bottom, leaving the top of the traps 1 metre above the surface of the water. The traps were then weighted to prevent escape of plant litter in windy conditions. The litter traps were located at two sites which typified the two main kinds of tree coverage (beech and common osier) of the lake margins mentioned previously. The litter traps were erected at distances of 1,3,5,7, and 10 m from the shore in straight lines and 3 replicate lines were set up at each location.

Litter samples were collected at weekly intervals and oven dried to constant weight, then converted to values per m*. Sampling took place between the 8th of October and the 11th of December after which no further leaf litter was caught by the traps. The litter traps were not set up earlier as they might have interfered with fly fishing on the lake and they had to be erected once the fishing season had ended. Observation of the trees suggested that a negligible quantity of leaf litter entered the lake before collecting started, and previous studies (Szcze- panski, 1965; Mason, 1970) suggest that the major proportion of autumnal litterfall occurs in the period October - December.

Results

Fig. 1 shows the total quantity of leaf litter (g m*) caught by the litter traps at the different distances from the lake margin at both sites during the period October - December. In both cases the quantity of litter shows an exponential decline with distance from the shore. These exponential regressions were tested by analysis of variance (Sokal& Rohlf, 1969) and found to be significant (y = 78.9 exp(-0.301x), F = 417.8, P < 0.001 for the common osier site and y=48.9exp(-0.311x), F= 120.8,O.Ol >P<O.OOl for the beech site).

The total quantity of litter entering the lake per metre of shoreline covered by trees was calculated from the areas under the curves using calculus; the values being 262. 1 g for the common osier site and

DISTANCE (METRES) Fig. I. Quantity of airborne plant litter caught by litter traps at different distances from the shore. Figure shows means and range of values for common osier site (-•-) and beech site (-0-).

157.2 g for the beech site. This gives a total of 73.9 kg of litter entering the lake or 1.026 g mm*.

Fig. 2 shows the pattern of litterfall at the two sites based on the total weekly collections. Both sites show a distinct period of maximum litterfall extending over 1 week during the last week of October and the first week of November. Leaf litter represented 95% of the litterfall, the remainder con- sisting of small twigs.

A - OCT- -NOVAL.EC-

Fig. 2. Weekly pattern of plant litterfall during the period October-December. Dark columns represent beech site, open columns common osier site.

Discussion

The quantity of deciduous plant litter entering Llyn Frongoch during the autumn of 1979, was calculated to be 73.9 kg. Deciduous plant litter is however shed throughout the year in the form of

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bud scales, flowers, twigs and fruit and the autum- nal litterfall may represent 70-80% of the total litterfall (Mason, 1970; Fisher & Likens, 1973; Gasith & Hasler, 1976). This would give a value of approximately 100 kg of airborne plant litter entering the lake each year. If 45% of the ash free weight of the litter is organic carbon (Westlake, 1963) then the annual litter input would amount to 45 kg. C. (0.625 g C mm*).

In comparison with phytoplankton production (15 g C.m-* yr’ Purdie and Jones unpublished data) in Llyn Frongoch, the airborne plant litter represents a small proportion of the organic carbon input. The input of deciduous plant litter into lakes however can vary considerably, and Table I shows a comparison between plant litter input and phyto- plankton productivity for several lakes of different sizes. In large lakes such as Lake Mikolajka and Lake Wingra litter input represents as little as 0.2% (Szczpanski, 1965) and 0.3% (Gasith & Hasler, 1976) respectively of the phytoplankton productiv- ity. In small lakes like Llyn Frongoch and Mirror Lake (Jordan & Likens, 1973) litter input repre- sents approximately 4-5% of the phytoplankton productivity. Assuming similar phytoplankton production and extent of afforestation of the lake margins, the relative importance of plant litter in comparison with phytoplankton increases as lakes become smaller as a result of a decrease in the ratio of surface area to shoreline length of the lakes (Gasith & Hasler, 1976).

Tab/e I. Comparison of airborne plant litterfall and phytoplankton productivity for lakes of different sizes (Lake Mikolujka: Szczpanski, 1965; Pieczynska, 1972b; Lake Wingra: Gasith & Hasler, 1976; and Mirror Lake: Jordan & Likens, 1975.)

Area Shoreline Percentage Phytoplankton Airborne Airborne (Ha) length afforestation productivity litter litter

(Km) (g C.mm2 yr-‘) (g C.m-’ yr-‘) (g mm’ wooded shoreline)

Lake Mikolajka 459 14.4 52 330 0.36 225

Lake Wingra 131 5.0 75 400 0.916 320

Llyn Frongoch 7.2 1.45 23 15 0.625 135

Mirror Lake 15 2.0 90 78 4.3 354

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Litter input into Llyn Frongoch nevertheless is low in comparison with Mirror Lake, as only 23% of the shoreline is planted with trees. Llyn Fron- goch also has a low litter input per metre of wooded shoreline (153 g C.m-2 yr -I) for the osier site and 92 g C.me2 yr-’ for the beech site) in comparison with that of Mirror lake (354 C.m-2 yr-l). This may be the result of the prevailing winds blowing most of the litter away from the lake, and the tree lined margin being sparsely wooded, At the beech site the trees are situated approximately 3 m from the shore and much of the litter will fall on this area between the trees and the water. Llyn Frongoch also has a low phytoplankton productivity in comparison with the other lakes as a result of an extremely low phosphate concentration (2.5 c(g 1-l Purdie and Jones unpublished data) which will increase the relative importance of litter input into the system. In such small oligotrophic lakes as Llyn Frongoch the establishment of highly afforested shorelines could increase the contribution of organic carbon from plant litter to as much as 25% of that represented by phytoplankton productivity.

The data presented here suggests that the majority of plant litter is deposited within the first 10 m from the shore and that litterfall follows an exponential decline with increasing distance from the shore. These results are similar to those described by Szczpanski (1965) and Gasith & Hasler (1976), though the decline in litterfall with increasing distance from the shore may vary from exponential to linear depending on local conditions (Gasith & Hasler, 1976). Therefore only the trees and shrubs in the immediate area of the shoreline make a significant contribution to the airborne litterfall entering the lake.

In small lakes with small outlet streams such as Llyn Frongoch, high rainfall can raise the water level several feet in a short period of time. The combination of high water and windy conditions may cause floating litter and litter accumulated at lake margins to be deposited along the shore when the water level subsides. This, together with the transportation of litter from the lake in streamflow, will reduce the quantity of litter available to the organisms with the lake; though no measurements of litter loss from the lake were made here. Pre- liminary results (Hanlon, in prep.) suggest that there may be an accumulation of organic material in the lake and some of the potential energy

entering the system is not readily being utilized by the leaf processing fauna and microflora. In addi- tion to organic material, plant litter contains nitrogen, phosphorus, potassium and trace ele- ments which may be limiting in the system. The contribution of these nutrients in litterfall, how- ever, will be small in comparison with that entering the lake in streamflow and seepage from ground water.

The planting of trees around lake margins has been considered as a method by which the quantity of organic carbon entering lakes could be increased. Results and discussion reported here suggest that such plantings, particularly around small oligo- trophic lakes with low surface area to shoreline length ratios, could provide an important source of organic carbon for the benthic invertebrate and microbial populations.

Acknowledgements

I would like to thank Dr. J. H. R. Gee for his help in erecting the litter traps and Dr. R. J. Wootton for his criticism of the, manuscript. This work was funded by a Davies Trust research fellowship.

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Received July 30, 1980.

Author’s address: R. D. G. Hanlon Dept. of Zoology The University of Wales Aberystwyth, Dyfed Wales SY23 3DA. U.K.