MEPS 156:51-66 (1997)  -  doi:10.3354/meps156051

Near-surface phytoplankton pigment from the Coastal Zone Color Scanner in the Subantarctic region southeast of New Zealand

Karl Banse*, David C. English**

University of Washington, School of Oceanography, Box 357940, Seattle, Washington 98195-7940, USA

Primarily based on satellite images, the phytoplankton concentration southeast (downstream) of New Zealand in the High Nitrate-Low Chlorophyll (HNLC) Subantarctic water between the Subtropical Convergence (STC) and the Polar Front (PF) is believed to be higher than in the remainder of the Pacific Sector. Iron enrichment is assumed to be the reason. To study the question, near-surface phytoplankton pigment estimates from the Coastal Zone Color Scanner for up to 7 yr were reprocessed with particular attention to interference by clouds. Monthly mean images were created for the U.S. JGOFS Box along 170°W and means for individual dates calculated for 7 large areas between 170°E and 160°W, 45° and 58°S, well offshore of New Zealand and principally between and away from the STC and PF. The areal means are about as low as in other HNLC regions (most values between 0.1 and 0.4 or 0.5 mg m-3, with very few winter images; median of seasonal means, 0.26 mg m-3) except at times near the STC. The higher means tend to occur in late summer and autumn. However, contrary to expectations, neither the PF nor the environs of the Subantarctic Front are distinguished by a zone of increased pigment. Also, of 24 spring-summer images of oceanic islands in mostly pigment-poor water, 17 yielded no recognizable elevated pigment; islands were 5 times surrounded by approximately doubled concentrations (ca 100 km in diameter), and 2 cases may have been associated with an extensive bloom. Inspection of offshore images showed concentrations of >=1 (up to 5) mg m-3 in rare patches of 65 to 200 km size on approximately one-tenth of the dates; such patches were not seen in Subantarctic waters of the eastern Pacific Sector. A case is made for Australian airborne iron supply being the cause that, presumably, would enhance large-celled phytoplankton. Since, however, the putative iron supply from the seabed around the oceanic islands or the near-by Campbell Plateau normally does not lead to phytoplankton increase, patches of neritic mesozooplankton advected from the shelves might be another mechanism that generates blooms of small-celled phytoplankton, but there are no data. These alternatives can easily be field-tested from concentration and size composition of the phytoplankton.

Chlorophyll · CZCS · HNLC region · Iron · New Zealand · South Pacific · Subantarctic

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