Role of aquatic macrophytes in trophic status of Northwestern Ontario lakes

Abstract

This research focusses on the development, refinement, and assessment of regional trophic status models for lakes of northwestern Ontario. Two companion papers describe the application of image analysis of aerial photographs as a technique for mapping aquatic plant distribution, and the design of an innovative sampling device to simplify the collection of macrophytes. Trophic status models were developed for lakes of northwestern Ontario, based on empirical relationships between Secchi disk transparency, total phosphorus, and chlorophyll a concentration. Corrective terms were added to the equations to adjust for the effect of water colour on Secchi disk transparency, and the effect of aquatic macrophyte abundance on the chlorophyll - phosphorus relationship. The adjusted models demonstrated an improvement in performance, as measured using standing stock of benthos as a response variable. These models would be expected to have applicability across the Precambrian Shield region of Canada. Distributional maps of aquatic vegetation were produced for area lakes using digital image analysis of aerial photographs. Recent improvements in the price and performance of computer hardware and software make this a viable alternative to the conventional, visual mapping technique. The maps produced are highly detailed, with differentiation to species possible in some instances. Certain species may be further partitioned into density classes. The authenticity of the maps depends on the ability to properly define the spectral 'signatures' for different macrophyte types. These signatures were most easily defined for floating-leafed and emergent forms; submersed vegetation proved more difficult to classify. The main detriment to this approach is the steep learning curve associated with the image analysis software. A thorough description and assessment of this technique is provided, with a discussion of its merits and deficiencies when compared with the conventional visual interpretive method. A portable macrophyte sampler was designed for use in the relatively inaccessible lakes of this region. As such, it was required to be lightweight, easily transportable, and useable by a single person. The device is effective for obtaining quantifiable biomass samples of most rooted aquatic plants over a wide variety of substrates and sampling depths. Details on the design, operation, and performance of the sampler are documented within.