Pinnularia walkerae A.C. Pereira & Torgan sp. nov. (Figs 18–21, 33–38)

Type: — BRAZIL. Amazonas: Tupé Lake 03 o 02'17,1''S, 60 o 15'45,3''W, 12 October 2003, Melo S., plankton, station 9 (Holotype HAS slide n o 6491! holotype specimen = Figure 18, row sample HAS 109584 ; isotype BR 4322!).

Valves lanceolate with weak to strong triundulate margins, protracted capitate ends (Figs 18–21, 33), valve length 65–85.4 µm, breadth 11.5–14.8 µm, length/breadth ratio is 5.65–5.77. Axial area 1/3 of breadth of valve, widening towards central area. Central area present, forming symmetric fascia. Raphe straight, filiform, proximal raphe endings bent in same direction (Fig. 34) terminal fissures sickle-shaped (Fig. 37). Terminal raphe endings curved towards secondary side of valves, while drop-like central pores deflect to primary side (Figs 34, 37). Striae (10–12 in 10 µm) radiate at middle, convergent at poles. Alveoli pattern with 3–7 rows of rounded areolae, each row with 7–8 areolae in 1 µm (Figs 35, 36, 38).

Etymology: —Dedicated to Dr. Ilse Walker, a researcher from the Instituto Nacional de Pesquisas da Amazônia.

Observations: — P. walkerae shows some similarities to P. egregia Metzeltin & Krammer in Metzeltin & Lange-Bertalot (1998: 173), P. polyonca (Brébisson) Smith (1856: 95) and P. rostratissima f. sub undulata Hustedt (1965: 397), whose features are described in Table 2, differences are the shape of poles and the striation pattern.

Pinnularia walkerae was observed in the plankton, during the low waters with 1.10 m depth, 1.0 m transparency, high temperature (32.9ºC), low pH (4.1) and conductivity (6.30 µS.cm -1).