BRAY: INFLUENCE OF WATER CURRENTS AND ZOOPLANKTON DENSITIES 



Even though the counts of plankton were stan- 

 dardized to densities, I attempted to sample the 

 same volume of water in each tow to make it 

 equally likely that relatively rare items would be 

 collected at both ends. In seven of eight collections, 

 volumes of water sampled did not differ signifi- 

 cantly between reef ends (^-tests, P>0.10); in 

 the last collection a significantly greater volume 

 of water was sampled at the west end of the reef 

 (P<0.01). The average length of the tows was 

 57.1 m, which corresponds to a filtered volume 

 of 11.2 m^. 



Small copepods (<4 mm carapace length) and 

 cladocerans were the most abundant items in 

 the samples, averaging 1,259/m^ and 836/m^. 

 Small copepods dominated in 93 samples while 

 cladocerans dominated in the remaining 55 sam- 

 ples. Most of the copepods were calanoids, al- 

 though cyclopoids were also present. The majority 

 of cladocerans appeared to be Evadne sp., but 

 Penilia sp. occasionally dominated. Larvaceans 

 ranked third in abundance, averaging 119. 6/m^. 



Densities of zooplankton differed markedly be- 

 tween the mcurrent and excurrent sample sites at 

 Naples Reef. For each collection, mean number 

 and dry weight of plankton pooled in excurrent 

 samples were lower than those in incurrent sam- 

 ples, regardless of the current direction. Differ- 

 ences in counts were significant in six of the 

 eight individual collections (Table 3), and for 

 all eight collections tested together (Wilcoxon's 



signed-ranks test, P<0.005). Estimates of dry 

 weights followed a similar pattern (Table 3). 



The trend of a decreased abundance near the 

 excurrent end was shared among many of the 

 zooplankton groups (Table 4). Cladocerans, lar- 

 vaceans, and bryozoan larvae were significantly 

 less abundant at the excurrent end in seven 

 of eight collections. Other groups were less 

 abundant near the excurrent end in some collec- 

 tions but not in others. For example, densities 

 of small copepods were significantly lower in 

 excurrent samples in six collections, but were 

 higher in another collection (Table 4). Overall, 

 mean densities of 7 of the 15 plankton groups 

 were significantly lower near the excurrent end. 

 Cladocerans, small copepods, and larvaceans 

 showed the greatest decrease near the excurrent 

 end, while polychaetes and nauplii averaged 

 slightly greater there. 



Foraging Experiments 



I attempted eight experiments between late 

 July and mid-December 1977; three were deleted 

 because several fish died in the cages. The follow- 

 ing analysis is based on the 27 of 31 fish in the 

 remaining five experiments that had food in their 

 guts and showed no signs of injury. The first four 

 experiments used larger individuals (117-214 mm 

 SL); the last experiment used smaller fish (88-117 

 mm SL). For each experiment, there were only 



Table 4. — Zooplankton densities near the incurrent and excurrent ends of Naples Reef, southern 

 California. Densities are averaged among means of the eight collection days. Columns to the right 

 indicate number of collections during which plankton densities at the incurrent end were greater, 

 less than, or equal to those near the excurrent end (Mann-Whitney [/-tests, two tailed; P«0.05). 

 Symbols next to incurrent densities indicate P values from a one-tailed Wilcoxon's signed-ranks 

 test for incurrent and excurrent differences in density among all collections combined. Plankton 

 groups are listed in order of decreasing differences in densities between incurrent and excurrent 

 ends of the reef 



Average number per m 



Number of collections 



Plankton group 



Incurrent 



Excurrent Incurrent < excurrent Incurrent > excurrent Incurrent = excurrent 



Cladocerans 



Small copepods 



Larvaceans 



Echinoderm larvae 



Doliollds 



Chaetognaths 



Medusae 



Large copepods 



Bryozoan larvae 



Fish larvae 



Decapod larvae 



Zoea 



Ostracods 



Nauplii 



Polyctiaetes 



1,097.2- 

 1,402.3ns 

 178.7" 

 54.6ns 

 41.8" 

 32.1- 

 39.8" 

 22.8* 

 15.5" 

 3.4ns 

 3.3ns 

 5.0ns 

 0.3— 

 1.0ns 

 0.9ns 



575.7 



1,114.8 



60.4 



15.7 



9,7 



16.9 



25.0 



9.7 



4.3 



1.3 



2.0 



4.4 



0.2 



1.2 



1.4 





 1 





 

 

 

 



1 







1 





 

 

 

 



1 

 1 

 1 



'2 



3 



2 



3 



'3 



'0 



5 



'3 



'5 



'1 



'2 



'3 



■PsO.05: "Ps:0.005; ns, not significant; - 

 ' Not present in all eight collections. 



• insufficient data; 



835 



