many jagged coralline "mushrooms" found on the reel Hats 

 (PI. 11 ). are probably remnants of former reef flats formed 

 when sea levels were several feet higher than present. 



Lagoon 



A notable attribute of Caroline's lagoon fromaconservation 

 perspective is its outstanding clarity and beauty. Throughout, 

 but especially near the Central Leeward Islets, the twisted reef 

 configurations studded with white sandy channels and deeper 

 circular openings generate a spectrum of stunning colors — 

 turquoise, apple green, tawny, azure, and royal blue (PI. 25). 



Caroline's lagoon. 8.9 km long, is closed. Though it 

 appears to dominate the atoll, its total area is less than that of 

 the combined reef flats. The lagoon is relatively shallow, 

 tapering in shape and depth at each end, and is crisscrossed with 

 living coral. Its bathymetry is unknown. 



In the north the lagoon is more sheltered, as the presence 

 of continuous vegetated land buffers the easterly trades. At its 

 northern extremity, merging reef flats squeeze the lagoon until 

 it disappears east of Pandanus Islet. A filled-in portion of the 

 former lagoon penetrates Nake for 300 m as a fishhook-shaped 

 mudflat, Sandy Inlet (Fig. 37. PI. 23). before succumbing to 

 encircling vegetation. At the lagoon's southern end, where 

 winds whip through the "blind channel." it is choppy, having 

 more sediment and slightly less visibility. However, w ithin the 

 lee of South Island' s north-central curve, the lagoon is frequently 

 quiet and reflective (PI. 24). 



Lagoon Hydrology : Although Caroline's hydrology has 

 not been studied, it has been closely observed for over two 

 years by Ron Falconer, especially the south end of the lagoon 

 and "blind passage.'" He has noticed that the lagoon water is 

 tv pically "perched" at a level above that of all but the daily high 

 tides. High tide water flows rapidly over the reef flats into the 

 lagoon but is held back by the reefs as the tide lowers. Ron has 

 noted that lagoon water at low tide is about 0.3 m higher than 

 water in the "blind passage." Water moves out of the lagoon 

 through a few channels that, although deep in places, form 

 broad, shallow troughs over the reef flats. A major channel 

 with a current flowing west at several knots passes along the 

 northwest point of South Island, although water passage is 

 impeded hv the reel flats west of South Island. If a channel 

 u ere to he blasted through the reef flats, .is has been proposed, 

 this delicate hydrolog) would be disrupted. For example, the 

 high tide w ater is never more than 20 cm above the coral heads 

 and reels m the lagoon. A man-made reef channel for vessels 

 could lower water levels 30-40 cm. thereby exposing and 

 killing the extensive Acropora Tridacna reefs within the lagoon. 



The "blind passage" northeast ol South Island (Fig. 50) is 

 sustained by a powerful northward How of water along the east 

 coast of South Island and a strong southward flow of water 

 along the seaward reels of Tridacna Islet. The South Island 

 flow is apparently augmented bv water draining from a large 

 shallow basin on the reef Hats south of the island. Water spills 

 into the "blind passage" and drams east at about 4 knots against 

 (he pre\ ailing Hade w inds and surf. There is much less current 

 at the west (inner) end of the passage, where less water is 

 collected, and throughout the passage at low tide i there being 

 essential!) no water How out from the lagoon). 



Patch Reefs : A complex series of patch reefs and coral 

 knolls ( primarily Acropora spp. i. circular and elongated, flank 

 the smaller motus and crisscross most of the lagoon's area (see 

 Chapter Frontispiece). They are particularly evident in the 

 southern two-thirds of the lagoon, where they approach and/or 

 break the surface. Coral limestone bedrock, surmounted by a 

 variety of living coral, mollusks. and other invertebrates, 

 provides their basic structure (Fig. 10). The atoll's perimeter 

 reefs shelter the knolls from storms, surf, and excessive erosion. 



Caroline's lagoon is gradually filling in with ever- 

 expanding patch reefs and debris washed in from the fringing 

 reefs. Since Arundel's time, the effects of detrital deposition 

 can be discerned as changes in the shapes of islets such as Nake. 

 Danger, and Arundel (compare Figs. 2 and 4 1. 



Such change is typical of atoll evolution. Geologically. 

 Caroline is a few steps behind one of its "neighbor" Line 

 Islands, Christmas, where sediments and coral growth have 

 converted the original lagoon into a maze of supersaline. 

 minilagoons and tiny islets, mostly cut off from the sea. Further 

 steps in evolution are exemplified by completely filled-in atolls 

 such as Jarvis and Vostok, where not even salty pools remain. 



Tridacna- Acropora Reef: Though the giant clam 

 (Tridacna maxima) is an abundant component of Caroline's 

 lagoonside reefs, exceptional aggregations flank the most 

 southerly windward motus (Brothers through Tridacna. 

 Figs. 44 to 48 ). Two especially outstanding reefs extend across 

 the lagoon from Tridacna to Ana-Ana (Fig. 10: PI. 26) and 

 Tridacna to Kimoa (Fig. 48). where Tridacna are attached to 

 Acropora spp. corals, a favored substrate (Braley. 1987). 

 Abundant inshore Tridacna on all these islets suggest that their 

 density is similar to that on the main reef: up to 20/. 25 nr 

 (i.e.. 80/m 2 ), averaging 35/nv for the entire area surveyed 

 (Sirenko& Koltun. Subchapter 1. 4. this volume). This exceeds 

 the highest densities known: up to60/nv at Reao Atoll. Tuamotu 

 Archipelago ( Richard. 1985). Densities of 6-20/m 2 , at Takapoto 

 Atoll (Tuamotus) are considered high. Throughout Caroline, 

 the clams averaged 18 x lOcminsize. Several species of giant 

 clams have suffered greatly in the Indo-Paeific from poaching 

 and ov erharv esling: few undisturbed populations exist ( Braley . 

 1987). Caroline is thus a special refuge for T. maxima. 



Lagoon Reel Flats : These varv considerably but are 

 narrower and more gently sloping than the seaward reef flats. 

 They are typically covered with fine coral gravel and coarse 

 sand In sheltered areas dower Long. Windward. Crescent. 

 South, upper end of lagoon), lush shrubbery — Cordia, 

 Toiirnefortia, Pisonia, Cocos — overhangs the lagoon. Here 

 fine silt. sand, and/or an algal slime are common (PI. 28). In 

 l l )88. narrow, sandy beaches occurred only on the north shore 

 of South (PI. 24) and east side of Shark (PI. 29). but in 1990, 

 sand was more common throughout Caroline. 



Where the lagoon shorelines arc less sheltered and 

 vegetation does not overhang the lagoon, unvegetated rubble 

 and sparse herb mats are typical. Here, lagoonside rubble 

 av erages less than 2 m wide ( PI. 3 1 ). This contrasts with their 

 seaw aid reef Hats, w Inch av erage 2 1 m w ide (PI. 12). 



Lagoon Reef Fauna: A Brief Summary : Caroline's 

 marine environment is rich vet essentially undocumented; 

 know ledge of its ecosystems is limited to small, outdated lists 



12 



