Chapter 20 



Lagoon Plankton of Eneivetak Atoll 



RAY P. GERBER 



Biology Department, St. Joseph's College 

 North Windham. Maine 04062 



INTRODUCTION 



The zooplankton community of atoll lagoons is 

 uniquely different from the zooplankton community in the 

 ocean waters outside the atoll for at least two reasons. 

 First, there is a greater abundance of zooplankton in the 

 lagoon, no doubt reflecting the better food conditions pro- 

 vided by exported reef detritus and phytoplankton produc- 

 tion (Gerber and Marshall, 1982). Second, the species 

 composition of the lagoon zooplankton largely consists of 

 species adapted to the restricted shallower conditions 

 found therein. Earlier studies on the zooplankton of atoll 

 lagoons were mostly concerned with determining abun- 

 dance, either to evaluate their food potential to corals 

 (e.g., Kramer, 1897; Russell, 1934; Motoda, 1938; Odum 

 and Odum, 1955; Johannes et al., 1970; Johannes and 

 Gerber, 1974) or in understanding the water exchange pat- 

 terns between lagoons and the surrounding sea (e.g., John- 

 son, 1949, 1954; Gilmartin, 1958; Michel, 1969; Michel 

 et al., 1971; Tranter and George, 1972). In these studies 

 little attempt was made to identify the organisms to 

 species. However, these works are important because they 

 provide a basic understanding of the ecology of zooplank- 

 ton in lagoon environments. For a more detailed discussion 

 on the ecological role of zooplankton in atoll lagoons see 

 Chapter 10 by Gerber and Marshall, Volume 1. 



Studies on the species composition of lagoon zooplank- 

 ton at Enewetak Lagoon and nearby atolls have for the 

 most part focused on the hoioplanktonic organisms, forms 

 that are permanently planktonic all of their lives; these 

 forms are the main attention of this chapter. Because infor- 

 mation is lacking on the life history of many of the species 

 mentioned, some meroplanktonic forms — organisms that 

 are temporarily planktonic — are no doubt included in this 

 checklist (Table 1). In the works discussed below, a variety 

 of sampling methods have been used to collect the zoo- 

 plankton, and each method has its inherent advantages 

 and disadvantages. In a sense it is perhaps fortunate that 

 no one method is best to sample the various types of zoo- 



plankton. One important aspect lacking in the data is ade- 

 quate seasonal samplings to provide information on how 

 species composition and abundance change over time. 

 Whereas most of the lagoon plankton studies were based 

 on samples collected over a few days in one season, the 

 present author has sampled more extensively, during two 

 winter periods and one summer period. 



DINOPHYTA 



Dinophyceae (Dinoflagellata) 



Nineteen species of dinoflagellates were identified from 

 plankton tows taken off the wide passage and/or deep 

 entrance at Enewetak during the summer of 1956 (Hirsh- 

 field et al., 1957). Their species list and nomenclature 

 were originally based on Lebour (1925) and Schiller 

 (1937). They have been revised here using Parke and 

 Dixon (1976). More recently, three additional planktonic 

 species were recorded from Enewetak at a mid-lagoon and 

 shallow station during two winters and one summer period 

 (Gerber, 1981). 



SARCODINA 

 Radiolaria 



Fifty species (including 18 undetermined) of radiolari- 

 ans were collected from plankton hauls made during the 

 summer of 1956 (Hirshficld et al., 1957) (Table 1). Gilmar- 

 tin (1958) and Gerber (1981) have recorded radiolarians in 

 plankton samples from Enewetak Lagoon but have not 

 attempted identification. Radiolarian fragments were 

 recorded in the gut contents of calanoid copepods from 

 Enewetak Lagoon (Gerber and Marshall, 1974). For 

 radiolarian classification see Campbell and Moore (1954). 



CILIOPHORA 

 Tintinnina 



Nine species of tintinnids were recorded from plankton 

 hauls taken in the wide passage and/or deep entrance at 

 Enewetak during the summer of 1956 (Hirshfield et al., 

 1957). Four of the species from Enewetak were con- 

 sidered to be new and are not known to have been 

 described. In February 1976, plankton tows in Enewetak 



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