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Recent studies of anadromous salmon (Oncorhynchus spp.) on the Pacific Coast of …

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- Nitrogen uptake in riparian plant communities across a sharp ecological boundary of salmon density

Site description

Our study sites comprise two old-growth western hemlock, salmon-bearing watersheds, the Clatse (52° 20.6'N; 127° 50.3'W) and Neekas rivers (52° 28.4'N; 128° 8.0'W), on the central coast of British Columbia, Canada (for detailed site description see [6]). Climate is cool and wet, with a mean annual temperature of 8°C and a mean annual precipitation of 3700 mm [44]. The two watersheds are separated by 30 km and each occurs in the Coastal Western Hemlock Biogeoclimatic Zone near the boundary of the central very wet hyper-maritime (CWHvh2) and sub-montane very wet maritime (CWHvm1) subzones [44]. Pink salmon (Oncorhynchus gorbuscha) and chum salmon (O. keta) are the predominant species of anadromous fish in these rivers. During the period 1990–1999, annual returns at Clatse River averaged 17,000 pink salmon and 5,000 chum salmon (22 salmon/m2), while at Neekas River, returns averaged 18,000 pink salmon and 30,000 chum salmon (23 salmon/m2) (Department of Fisheries and Oceans escapement data: 1990–1999). All spawning activity occurs from the estuary upstream to a 5 m high waterfall that excludes further upstream migration of the salmon at both watersheds. The falls occur 1 km upstream on the Clatse River and 2.1 km upstream on the Neekas River.

Sample collection and processing

Foliar samples were collected from seven species of riparian plants above and below the waterfalls at Clatse and Neekas watersheds in early September 1999. These comprise deerfern (Blechnum spicant), false azalea (Menziesii ferruginea), devil's club (Oplopanax horridus), salmonberry (Rubus spectabilis), Alaskan blueberry (Vaccinium alaskaense), red huckleberry (V. parvifolium) and western hemlock (Tsuga heterophylla). Collection sites were within 50 m of the top and 50 m of the bottom of waterfalls along the stream, and within 15 m of the stream edge perpendicular into the forest. At each site foliage from up to nine individual plants of each species were collected.

All foliar samples were dried in an oven at 67°C for three days and individually ground to a fine powder in a Wig-L-Bug Amalgamator (Crescent Dental Manufacturing Co.). Total nitrogen and N isotope analyses were conducted at the Stable Isotope Facility, University of Saskatchewan, using an automatic Nitrogen/Carbon analysis (ANCA) gas/solid/liquid preparation module with combustion tube temperature set at 1000°C, coupled to a 20/20 mass spectrometer (Europa Scientific Inc.). Measurement precision is approximately +/- 0.35‰. Natural abundance of 15N (δ15N) is expressed as the deviation from the standard, atmospheric N2, in parts per thousand (‰) and calculated as:

1) (Rsample/Rstandard - 1) × 1000

where R is the ratio of 15N/14N stable isotopes.

Estimating %MDN

We converted δ15N values to %MDN using the equation (modified from [12]):

2) %MDN = [(Obs-TEM)/(MEM-TEM)] × 100

where Obs is the δ15N value of the sample from a site below the waterfalls, TEM is the δ15N value of the terrestrial end member (foliar sample of same species above falls), and MEM is the δ15N value of the marine end member (salmon tissue). We use 13.01‰ for MEM [41].

Vegetation community inventory

In the late summer of 2000 and 2001, we inventoried riparian vegetation on the Clatse and Neekas rivers in small plots (10 m × 10 m) above and below the waterfalls at both Clatse (four plots below falls, three above falls) and Neekas (three plots below falls, four above falls) rivers. We matched as closely as possible the forest structure, canopy cover and slope in the sites above and below the falls and as such, plot locations ranged from 100 to 250 m above and below the waterfalls and were within 20 m of the stream. All plots had negligible slope. Within each plot, understory vascular plant species were identified and percent cover of each visually estimated. Canopy cover was similar above and below the falls on both watersheds (Clatse above falls: 1472-6785-3-4-i1.gif = 58.0+/-5.6%; Clatse below falls: 1472-6785-3-4-i1.gif = 66.5+/-3.5%; Neekas above falls: 1472-6785-3-4-i1.gif = 48.8+/-9.0%; Neekas below falls: 1472-6785-3-4-i1.gif = 49.0+/-11.4%). While the Neekas River has no history of commercial logging, Clatse River had some old-growth removal in the early 1940's, and currently a mixed old-growth-second growth western hemlock forest dominates the plots in this location [58].

Soil profiles were dug in representative sites to determine the humus type and original parent material. Mor humus forms predominated on the Clatse and Neekas rivers above the falls with matted LFH (decomposing litter) horizons ranging from 10–25 cm thick. Below the falls, ligno, lepto and mull moder humus forms predominated with a thin LFH 3–10 cm thick and an Ah (mineral horizon enriched in organic matter) 5–20 cm thick [58]. The parent material at all plots was dominated by coarse alluvial sand interspersed with a few large rock fragments. Morrainal silt deposits increased with increasing distance from the stream.

Data Analysis

Sources of variation in foliar 15N and total %N among and within watersheds, above and below the waterfalls and among plant species were compared (paired-t tests, two-way ANOVA, multiple range tests). Understory plant species were grouped into soil nitrogen indicator categories (poor, rich) based on Klinka et al. [29] (Table 1). Unclassified and nitrogen-medium soil indicator species were not included in the analysis. On each watershed, we compared total cover for each indicator group above and below the waterfalls. As assumptions of normality and homoscedasticity were not met in all cases, we used Mann-Whitney non-parametric tests.

Authors' Contributions

DDM carried out the foliar sampling and statistical analysis of total %N and N isotopic data. MDH carried out the design and analysis of the vegetation community data. TER conceived of the study and participated in its design and coordination. All authors contributed to writing the ms but DDM was primarily responsible for the first draft. All authors read and approved the final manuscript.


We would like to thank Tracy Rennie, Samantha Robbins, Jonathan Moran, Gilbert Ethier, Barbara Hawkins, Dan Klinka, Bristol Foster, Chester Starr, Carsten Brinkmeier, Danny Windsor, Mike Windsor, Gerry Allen and Joe Antos for field assistance or discussion, Myles Stocki for stable isotope analysis, and Barbara Hawkins and Richard Ring for additional lab space. We would also like to thank the Blue Fjord Charters, Larry Jorgenson, the Raincoast Conservation Society and the Heiltsuk First-Nations for additional field support. This project was supported by funds from the David Suzuki Foundation, Friends of Ecological Reserves and a Natural Sciences Engineering Research Council operating grant (N2354) to TER.

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