Pinnularia nelsonii A.C. Pereira & Torgan, 2014

Pinnularia nelsonii A.C. Pereira & Torgan sp. nov. ( Figs 7–12 View FIGURES 2–21 )

Type: — BRAZIL. Amazonas: Tupé Lake 03 o 01'33.5''S, 60 o 15'58.0''W, 16 December 2003, Melo S., plankton, station 1 (holotype HAS slide n o 6493! holotype specimen = Figure 7 View FIGURES 2–21 , row sample HAS 109583 GoogleMaps ; BR 4320 ! Isotype GoogleMaps ).

Valves linear with parallel margins, acutely rounded ends, valve length 36–47.5 µm, breadth 11–12.7 µm, length/ breadth ratio 3.27–3.74. Axial area with 1/5 width of valve, slightly widening towards central area. Central area present, generally forming symmetric fascia. Raphe straight, filiform, proximal raphe endings bent in same direction, terminal fissures sickle-shaped. Striae (13–14 in 10 µm) radiate at middle, parallel to slightly convergent at poles ( Figs 7–12 View FIGURES 2–21 ).

Etymology:— Dedicated to our colleague Dr. Edinaldo Nelson dos Santos-Silva, whose work has improved the quality of life in rural Amazonian populations.

Observations:— P. nelsonii resembles P. acuminata var. interrupta (Cleve) Patrick & Reimer (1966: 623) , P. instabiliformis Krammer & Metzeltin in Metzeltin & Lange-Bertalot (1998: 177) and P. wisconsinensis Camburn & Charles (2000: 29) ; but there are several features that distinguish P. nelsonii from these species (see table 2). P. acuminata var. interrupta differs by a broader axial area and lower striae density. P. instabiliformis , described by Metzeltin & Lange-Bertalot (1998) from Calado Lake in the Amazonian region, also differs from P. nelsonii by the axial area shape, striae pattern and density and by the lateral raphe structure. P. wisconsinensis has a distinct valve length (up to 60 µm), subrostrate ends, oblique raphe and different striation pattern and it has a lower striae number in 10 µm (below to 11).

Pinnularia nelsonii was rarely observed in the plankton, during the low waters with 1.0 m depth, 0.70 m transparency, high temperature (30.7ºC), low pH (4.6) and conductivity (5.70 µS.cm -1).