more restricted distributions, e.g., Codium fragile 

 only at BP, and Alcyonidium spp. and Tuhularia 

 crocea only at effluent sites during 2-unit opera- 

 tion, or Mycale fibrexilis at ES, and Balanus 

 halanoides at ambient sites during 3-unit opera- 

 tion. Balanus balanoides is typically an intertidal 

 barnacle; its presence on subtidal panels is unpre- 

 dictable. For example, Battelle researchers 

 (1968-1978) found 65-70% cover of/?, balanoides 

 in 1970 and 1971 at GN, and less than 2% in all 

 other years (Battelle, unpublished data). 



Numerical abundance. Of the 22 species prev- 

 alent in terms of percentage of cover, seven were 

 assessed by counting the number of individuals 

 per panel: Balanus amphitrite, B. balanoides, B. 

 crenatus, B. ehurneus, B. improvisus, Balanus ju- 

 veniles, and Mylilus edulis (Table 3); these organ- 

 isms occur as individuals, and are most likely to 

 influence woodborer attack. Generally, numerical 

 abundance data support conclusions drawn from 

 percentage cover, and demonstrate similar distri- 

 butions, e.g., Balanus crenatus was the most abun- 

 dant fouling organism, B. improvisus was more 

 abundant during 2-unit operation than 3-unit op- 

 eration, and was most abundant at ES, and B. 

 balanoides was found only during the Feb-Aug 

 period during 3-unit operation. In some cases, 

 general patterns can have a single probable cause. 

 For example, the lower abundances of juvenile 

 Balanus in the Aug-Feb 'after' period, and the 

 higher abundances in the Nov- May period are 

 attributable to a slightly later set during this first 

 year of 3-unit operation. If these juveniles were 

 primarily B. crenatus, it would explain the gener- 

 ally lower densities for B. crenatus during the 

 Nov-May 'after' period (i.e., individuals, settling 

 late, were too small to be identified in May). 



Wood-boring Species 



Numerical abundance. Most of the wood de- 

 composition (both naturally-occurring and in 

 man-made structures, e.g., docks and wooden 

 lobster pots) that occurs in local waters is the 

 result of tunneling and feeding by shipworms. 

 Teredo spp. Therefore, one measure of the inten- 

 sity of potential wood-loss is the numerical abun- 



dance of teredinids. Counts of the native species 

 of shipworm. Teredo navalis, were highest in pan- 

 els exposed from May to November (Fig. 6), and 

 ranged to a maximum of 300 per panel. This 

 exposure period encompasses the entire settlement 

 period (roughly July-September, Graves 1928; 

 Ilillman et al. 1985), and densities were higher 

 than in Aug-Feb or Feb-Aug exposure periods, 

 which included only part of the settlement season. 

 Settlement of T. navalis did not occur during the 

 Nov-May exposure period. 



Seasonal patterns of shipworm abundance are 

 also seen in densities of Teredo juveniles (Fig. 7). 

 At ambient water sites (GN, FI, WP), young 

 shipworms ( < 5 mm; too small to be identified 

 to species) were common only in the Feb-Aug 

 period, indicating recent settlement. At the 

 effluent sites (EB, ES), Teredo juveniles were 

 found in each exposure period; however, these 

 were presumably young Teredo bartschi, a non- 

 native shipworm with an extended reproductive 

 season. Adult T. bartschi have occurred in the 

 MNPS effluent since 1975, and were particularly 

 abundant (100-150 per panel) in Aug-Feb and 

 May-Nov exposure periods (Fig. 8). 



Comparisons of abundance during 2 -unit and 

 3-unit operation, for each category of woodborers, 

 are presented in Table 4. Of particular interest 

 are the increased densities of T. bartschi at the 

 effluent sites, and the increased densities of T. 

 navalis during the May-Nov exposure periods at 

 WP and FI (the sites most likely to be influenced 

 by the 3-unit plume). Densities of the wood- 

 boring crustaceans, Limnoria spp. and Chelura 

 terebrans, although variable in time and space, 

 show little direct relationship to operational his- 

 tory or to levels of wood-loss. For example, 

 densities of Limnoria and Chelura at WP decreased 

 during 3-unit operation, when wood-loss in- 

 creased, and densities of Limnoria at GN were 

 higli during the Nov-May exposure period, when 

 wood-loss was negligible. 



Percentage of wood-losss. The amount of wood 

 lost from panels is closely related to the numerical 

 abundance of shipworms. Wood-loss varied 



Exposure Panel Program 237 



