Population Ecology Studies on Infaunal Polychaetes of Pauatahanui Inlet

Abstract

Macrofaunal polychaete densities across a stable, fine-sand, intertidal flat in Pauatahanui Inlet were examined from a set of seasonal samples, 500 μm sieve processed, from a stratified transect pair. Density Patterns had shore-normal trends, despite apparently weak tidal-cycle environmental gradients. Zones of high abundance of common species persisted unchanged, but seasonal increase and decline occurred within them, with also some population redistribution attributable to differential mortality/recruitment, or possibly to migration. Correlation analysis did not detect interspecies relationships linked to the abundance and zonation of the common polychaetes, all deposit-feeders. The population dynamics of six species was investigated from the transect-pair seasonal samples, supplemented by subsequent more finely-sieved samples during dense recruitment periods. The maldanid Axiothella serrata had three identifiable age groups O, I, and II+, with I plus II+ density to about 550 m-2. The new O group began to appear in October-November, as aggregates below 3 cm sediment depth, believed to represent lecithotropic, direct-developing, siblings from egg masses of the II+ group. Juvenile setiger-total frequencies indicated synchronous adult spawning occurred at varying intervals during an approximate six month period. Peak density of dispersed, near-surface recruits reached 29 thousand m-2. Setiger-total was the most sensitive indicator of size and age in juveniles of up to about 15 setigers. Zonation patterns were age specific. The predominantly lower-shore capitellid, Heteromastus filiformis had a short summer spawning period with settlement ending before May, when population density was up to 10 thousand m-2. Merger of O group into the adult size range occurred in about one year, and probably first spawning was at the end of the second year, with life span of three years or more. The nereidid Nicon aestuariensis had I+ and older age groups at barely detectable densities. Spawning was probably in late summer although an O group, at about 500 m-2, was not detected until May. The spionid Scolecolepides benhami had apparently unchanging size structure and density (about 400 m-2 transect-wide); new settlement was not detected. High density occurred only in a narrow near-shore strip. Nicon aestuariensis also declined down-shore, but more gradually. The spionid Microspio sp. and capitellid Capitella sp. were short-lived, near-surface species, with apparently continuous recruitment from planktotrophic and direct-development respectively. A spring recruitment increase created at first a separate modal group of juveniles, and raised Microspio sp. density to 45 thousand m-2 and Capitella sp. to 7 thousand m-2. Capitella sp. declined in density downshore, but Microspio sp. was only weakly zoned. The common polychaetes had largely concordant density cycles with settlement (or peak settlement) spring-summer orientated, although 500 μm mesh processing detected the peak of surviving adults in autumn-winter. Pauatahanui polychaetes as an assemblage, life history traits, links between population structure and zonation, and problems in polychaete population studies are discussed.