Five different substrates (sand, fine gravel, coarse gravel, cobble and boulder) in nine streams forming a trophic gradient in the Ottawa Valley have been sampled in June 1992 to determine the effect of trophic level and substrate size on benthic communities. Total biomass of invertebrates (B, mg dry mass m$\sp{-2})$ and periphyton are strongly related to substrate particle size $(\Phi$, $-$Log$\sb2$diameter in mm) and trophy, represented by total phosphorus concentration (P, $\mu$g L$\sp{-1}).$ The model for periphyton biomass (Log$\sb $Peri = $-$0.08 $-$ 1.68Log$\sb $P + 1.08Log$\sb $P$\sp2\ -$0.13$\Phi\sp2)$ explains 67% of the observed variability and the model for invertebrate biomass (Log$\sb $B = 2.69 + 0.55Log$\sb $P + 0.3Log$\sb $Peri $-\ 0.01\Phi\sp2\ -$ 0.02Z) explains 68% of the observed variability (Peri = periphyton biomass in mgChla m$\sp{-2}$ and Z = depth in cm). The highest invertebrate biomass is observed on intermediate-size substrate (gravel) in phosphorus-rich streams. Periphyton biomass peaks on coarse substrates (boulder and cobbles) in streams where total phosphorus concentration is high $(>$50$\mu$gL$\sp{-1}).$ Individual body mass (W$\sb{\rm moy})$ is not affected by substrate size but is positively correlated with total phosphorus concentration (Log$\sb $W$\sb{\rm moy}$ = 1.67 $-$ 1.71Log$\sb $P + 0.96Log$\sb $P$\sp2,$ R$\sp2$ = 0.44). The effect of phosphorus and substrate on total biomass and mean individual body mass, are reflected in the size distribution of the invertebrates. Although the shape of the distribution is constant (unimodal), abundance of medium and large organisms increases in phosphorus-rich streams on intermediate-size substrates. A polynomial model including phosphorus concentration and substrate particle size is presented to quantify those variations of the size spectra. The predictions of this model are close to the observed values (R$\sp2$ = 0.89, residual mean square = 0.075) and this model could be very useful for a fast estimation of invertebrate size spectra.

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Two studies on the effects of enhanced ultraviolet-B radiation (UVB; 280--320 nm) on planktonic organisms of Canadian High Arctic lakes are presented. In the first study, the long-term effects of a moderate increase in UVB levels on the planktonic community of a lake were evaluated using in situ mesocoms. Four mesocosms (3 m square and 3 m deep) were placed in Two Basin Lake, a small lake (14.2 ha) located on Ellesmere Island (79°55.5'N, 84°40'W; Nunavut, Canada). For 27 days, two mesocosms were exposed to full sunlight (including ambient UVB) while two others were exposed to sunlight plus artificially enhanced UVB. Chlorophyll a, zooplankton mean length and carbon allocation into macromolecular constituents were not affected by enhanced UVB. Phytoplankton productivity displayed diverse and inconsistent responses to enhanced UVB. Picocyanobacteria abundance decreased in the enhanced UVB mesocosms, but only at the surface. Enhanced UVB generally increased heterotrophic bacterial abundance and activity. Heterotrophic nanoflagellates and zooplankton abundances increased in the enhanced UVB treatment after 14 days. The cladocerans and rotifers were positively affected by UVB, while the copepods were negatively affected. The high levels (≥ 5 mg L -1) of dissolved organic carbon (DOC) in this lake combined with vertical mixing generally protected the planktonic community from direct damage by enhanced UVB. It is hypothesized that UVB may have indirectly stimulated the microbial food web and the rest of the food chain through increased photodegradation of high molecular weight refractory dissolved organic matter into more bioavailable nutrients. In the second study, the short-term (2 to 24 h) effects of enhanced UVB on carbon uptake rates, photosynthetic fractionation into three size classes (picoplankton [0.2-2 mum], nanoplankton [2-20 [mum] and netplankton [> 20 mum]) and carbon assimilation into the four main end-products (low molecular weight metabolites [LMW], lipid, polysaccharide and protein) were assessed for nine lakes located near Resolute (74°15'N, 94°50'W) on Cornwallis Island (Nunavut, Canada). These lakes have low DOC levels (≤ 2 mg L-1). For each lake, 14C-inoculated water samples were exposed to 6, 25, 50 and/or 100% surface irradiance levels (E o) under natural solar radiation (including ambient UVB) or solar radiation plus artificially enhanced UVB. (Abstract shortened by UMI.)

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Source: Dissertation Abstracts International, Volume: 34-04, Section: B, page: 1574. / Thesis (Ph.D.)--The Florida State University, 1973.