3.4 Zooneuston of the Tropical Pacific 



YUZENALY P. ZAITSEV. LEONID N. POLISHCHUK, and BORIS G. ALEXANDROV 



Department of Active Marine Surface Hydrobiology, Institute of Southern Seas Biology, Odessa Branch, USSR 



Introduction 



Seventy-nine zooneuston samples were taken at 23 stations 

 in tropical areas of the Pacific Ocean (Table 1 ). The sampling 

 equipment consisted of a PNS-2 two-tier plankton-neuston net 

 and an MNT fry-neuston trawl (Zaitsev, 1 97 1 ). The PNS-2 net 

 (mouth area 0.1 nr; mesh size 1 50 urn) permitted simultaneous 

 manual skimming of the upper surface of the upper layers of the 

 pelagial (i.e., of neustal [0-5 cm] and subneustal [5-25 cm]). 

 The MNT trawl (moutn area 0.39 m 2 ; mesh size 350 p.m) made 

 possible high-speed catching of mobile neustons at a vessel 

 trawling speed of 3 m/s. The average skim duration was 

 10 min. 



To facilitate the study of neuston community formation, 

 the whole of the ocean area investigated was subdivided into 

 three types of zones, namely 1. atoll lagoons (shallow water; 

 complete or partial isolation from the ocean; developed littoral 

 communities); 2. semienclosed marginal seas (relatively little 

 exchange with the ocean; mostly shallow water; well-developed 

 pelagic-shelf communities); and J. open-ocean waters (pelagic 

 communities predominant). The sampling effort covered an 

 extensive portion of the tropical Pacific from Caroline Atoll in 

 the east (154°51'E). The latitudinal limits of station locations 

 were 1 1°20'N and 8°40'S. 



TABLE 1 



Neuston sample collection in the tropical Pacific. 



The neuston (surface plankton) of the tropical Pacific has 

 been described in publications of a general character (Geinrikh. 

 1 964 ). There are also papers on the distribution of its individual 

 components (e.g., of copepods of the Pontellaidae family 

 [Sherman. 1963; Voronina. 1964] and oceanic water striders of 

 the genus Halohates [Herring, 1961]). Results of pleuston 

 studies are reported in a major survey paper by Savilov ( 1 969). 



The need for the present study was dictated by a considerable 

 hiatus in observations, as well as by the scarcity of quantitative 

 data on neuston distribution in conditions of increasingly 

 widespread and intense pollution of the World Ocean. 



The zooneuston of the tropical Pacific found during the 

 present studies was represented by epineuston consisting of 

 oceanic water striders (Halohates) as well as by hyponeuston: 

 copepods of the family Pontellidae, marine snails of the genus 

 Janthina, larvae of benthic invertebrates, and fishes. In addition, 

 allowance was made for the usual neuston components in the 

 form of semisubmerged organisms: Physalia (Siphonophora), 

 Velella velella, and Porpita pacifica (Chondrophora). 



Analysis of the quantitative distribution of organisms 

 leads to some basic conclusions regarding the distinctive 

 features of neuston in the types of water areas studied. 



The average counts of animals in the open ocean and in the 

 South China Sea were comparable (2,730 and 3,160 ind/m 3 ), 

 while the concentration of animals in the lagoon at Caroline 

 Atoll was 50 times lower. Larger organisms (medusae, salps, 

 fish larvae and fry , euphausids hyperiidea, et cetera) constituted 

 aconsiderable fraction of the total neuston biomass, accounting 

 for as much as 2.5 g/nV (Station 1 19) and sometimes attaining 

 values an entire order of magnitude higher ( Station 128), where 

 Porpitae were dominant. 



The relative neuston content was richer in Caroline Atoll' s 

 lagoon, the reverse of that for animals overall (Table 2). The 

 low numbers of oceanic animal species in the lagoon against a 

 background of early developmental stages of benthic 

 invertebrates (Decapoda, Gastropoda, Cirripedia, et cetera) 

 were due to shallowness of the waters and their relative 

 isolation from the open ocean. The distinctiveness of the 

 individual sea area categories (atoll, marginal sea. open ocean) 

 was clearly evident from the proportion of benthic animal 

 larvae in the total neuston count. Their percentage content 

 decreased steadily from its maximum value of 99% in the 

 lagoon to 75% in the South China Sea and 46% in the open 

 ocean. The epineustonic Halohates, which breed along coasts, 

 were similarly distributed, while the Pontellidae distribution 

 behaved inversely (0.24% and 54%, respectively). 



In addition to benthic-invertebrate larvae, the neustal also 

 contained large numbers of pelagic animals at early stages of 

 ontogenesis, which validates calling neuston "the ocean's 

 breeding ground." 



Comparison of the numbers of organisms in the microlayers 

 investigated confirmed the existence of conditions conducive 

 to neuston growth in all of the water areas studied. The sole 

 exception to this rule was the waters of the Marianas sector 

 (Stations 121-126) (Table 3). 



223 



