Diversity and distribution of spine-bearing species of <i>Pinnularia</i> in eastern North America

Six species of large-celled <i>Pinnularia</i> Ehrenberg from lakes of eastern North America, <i>P. dariana</i> (A.W.F. Schmidt) Mills, <i>P. alabamae</i> Krammer, <i>P. bihastata</i> (A. Mann) R.M. Patrick, <i>P. torta</i> (A. Mann) R.M. Patrick, <i>P. rexlowei</i> sp. nov. and <i>P. spinifera</i> sp. nov. are investigated with light and scanning electron microscopy. All six species are shown to possess marginal spines, a trait sporadically encountered in <i>Pinnularia</i> and generally rare in raphid diatoms. Besides having spines, the six species differ in their frustular morphology, including appearance and position of spines. We summarize data on their ecology and distribution using a set of 702 sediment core samples from 424 lakes from eastern USA and Canada. <i>Pinnularia rexlowei</i> is only known from Quaternary lacustrine deposits in the State of Connecticut and from the surface and subfossil sediments of two lakes in northeastern USA. The other five species are relatively common, and at least one species of spine-bearing <i>Pinnularia</i> was found in 34% of studied lakes. <i>Pinnularia dariana</i> and <i>P. torta</i> reach as far north as Labrador, while <i>P. alabamae, P. bihastata</i> and <i>P. spinifera</i> were only encountered south of Canada. <i>Pinnularia alabamae</i> and <i>P. spinifera</i> are characteristic for lakes with relatively soft and slightly acidic water, while <i>P. bihastata, P. dariana</i> and <i>P. torta</i> are found in a wider range of physico-chemical conditions, including alkaline and eutrophic lakes.