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Additional aquatic species.

Values from several amphibians, protozoans and other aquatic species are also available. Studies of the frogRana pipiensand the toadBufo fowleriwere considered reliable with restriction with NOEC values of 7.5 and 41 mg-B/L, respectively. Studies of development of the salamandersAmbystoma jeffersonianandAmbystoma maculatumand the frogRana sylvaticashowed effects at the lowest exposure tested (LOEC 49.5 mg-B/L). Several protozoan studies are considered reliable with restriction due to limited information about the tests and results. However, values fo rParamecium caudatum, Opercularia bimarginata, Uronema pardaczi, andEnterosiphon sulcatumare available with chronic endpoints ranging from 10 to 30 mg-B/L.

Multi-species studies

A number of multispecies studies have been carried out (Guhl, 1992a, 2000). In a laboratory microcosm test using abundance and presences of prokaryotes and micro-eucaryotes of six trophic stages, the NOEC for borate was found to be 2.5 mg-B/L and LOEC of 5 mg-B/L.A laboratory river model, consisting of sequence of several vessels fed a mixture of treated wastewater and drinkingwater was monitored for biotic indices of the prokaryotes and micro-eucaryotes. No adverse effect was found at 1 mg-B/L so the threshold for effect is greater than 1 mg-B/L. Studies of outdoor ponds with up to 29 species over two years showed no significant difference when treated with 0.7 mg-B/L. Field studies in outdoor ponds over two vegetation periods showed no toxic effects of borate at concentrations between 0.16 and 1.52 mg-B/L.

Field Studies

Awareness of the early trout studies led to studies of wild trout population data and these reviews suggest that boron is not very toxic to wild trout where boron occurs naturally. Loewengart (2001) pointed out that nearly half of streams in California (USA) with viable populations of wild trout have boron concentrations equal to or above 0.1 mg-B/L. One stream (Little Warm Springs Creek, California) had a boron concentration of 13 mg/L. The Firehole River (Wyoming, USA) has a world-renowned trout fishery, even though it has elevated boron concentrations. The river receives geothermal input from geysers and hot springs, so has warm waters as well as elevated boron with variations over space and time in the water system. Studies of trout reproduction in the Firehole River system reported that trout delayed spawing until winter, presumably to reduce stress from high summer temperatures, but in doing so, reproduce successfully in stream areas where boron is highest, ranging from 0.4 to 1.2 mg-B/L (Meyer et al. 1998). Of note here is that the boron concentration encountered by the trout is variable, both seasonally and spatially. While the trout population may be considered to be adapted, it has not adapted to a single background concentration, but rather to a range of values.

Barros (2005) investigated rainbow trout condition and reproduction in the Rio de los Patos and Rio Agua Caliente in Argentina. This population was introduced into this river system in 1969 and has maintained itself since then. This study demonstrated that rainbow trout reproduces and maintains abundant populations in the range of 1.0 to 17.0 mg-B/L and 16.9 to 27.1 mg-B/L in the two rivers, respectively. Boron concentrations vary in this system, so mobile individuals may encounter varied exposures. Nesting sites (redds) were limited by the presence of suitable substrate, not by boron concentration.

Similar results are reported by Guhl (1992a) for other trout species in German surface waters and hatcheries, with trout populations in waters of 0.8 to 1.2 mg-B/L (Schilling lake in upper Bavaria) as well as in waters of 0.1 mg-B/L (Taubergiessen area in southern Baden) and in hatcheries with 0.01 to 0.08 mg-B/L (Albaum and Lohmar facilities). These studies provide an additional line of evidence regarding environmentally acceptable boron concentrations.