The breakdown of Picea mariana (Mill.) BSP. (black spruce) needles in a recently created northern reservoir, Southern Indian Lake, Manitoba

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Crawford, Patrick John
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Formation of the Southern Indian Lake reservoir, northern Manitoba, added an estimated 5.4 x 10/5 tonnes of Picea mariana *(black spruce) needles to the lake. The breakdown and macroinvertebrate colonization of needles in the lake was followed by stringing needles on monofilament line, placing the strings into 3 mm-mesh bags, and situating the bags along four shorelines representing different shoreline types (clay vs bedrock) and wave exposures (highly exposed vs protected). Sampling was done in 0-41 d and 328-384 d phases. Effects of excluding macroinvertebrates were tested by using needle strings placed in 50 um-mesh bags at the clay low exposure shoreline. Needle breakdown occurred in two stages. Initial weight losses were primarily due to leaching and microbial conditioning. Subsequent weight losses were primarily due to macroinvertebrate feeding and/or wave action. Over the entire 384 d period, leaching and microbial conditioning each accounted for ~30% of total weight losses. Macroinvertebrates accounted for ~40% of the total weight losses. High wave energies increased weight losses by as much as 18%, while heavy sediment deposition decreased weight losses by up to 30%. Processing coefficients (k) for the entire 384 d ranged from 0.0011 d-1 for the 50 um-mesh packs to 0.00097 d-1 for the 3 mm-mesh packs at the bedrock high exposure shoreline. Macroinvertebrate colonization of needle strings occurred rapidly (>3 d) and was mainly by chironomid larvae. Colonization was due to macroinvertebrates present both within the surrounding bottom substrate and drifting within the water column. Most chironomid larvae colonized the surface of the needles, however, Phaenopsectra punctipes (Wied.) and Brillia flavifrons (Joh.) larvae appeared to mine into the mesophyll region of the needles and consume them from inside. Heavy wave action, heavy sediment deposition, and the reduction of needle surface area as breakdown proceeded reduced the numbers of colonizing macroinvertebrates and caused large shifts in the structure of colonizing chironomid communities. P. punctipes was the dominant colonizer at all shorelines, except those with heavy sediment deposition. At such shorelines, Microtendipes and Procladius were dominant. Values of k for Southern Indian Lake most resemble those for conifer needle breakdown in streams, indicating the significance of wave action in the lake. The breakdown of P. mariana needles in Southern Indian Lake appeared to be a significant source of carbon during the year following impoundment, and needles may have been an important habitat for macroinvertebrates within localized areas.