Rhodoliths in the Gulf of California: Growth, demography, disturbance and effects on population dynamics of catarina scallops.

摘要

Rhodoliths are unattached, branching, crustose coralline red algae that create biogenic habitat for diverse benthic communities. Due to increasing anthropogenic disturbances, I examined the population biology, growth, and community ecology of rhodoliths in Bahia Concepcion (El Requeson) and Isla San Jose (Diguet), Mexico. In studies of populations of Lithophyllum margaritae and Spongites trichotomum, I found that both the El Requeson and Diguet populations were dominated by small (3cm dia.) individuals. However, density (9,000 vs. 400/m2) and size distribution (1–4 cm vs. 1–20 cm) varied between populations, respectively. In field growth studies using Alizarin staining I found L. margaritae growth at 8m at El Requeson varied seasonally (summer −9.9 ± 2.2 vs. winter −1.7 ± 0.3 mm*yr−1 diameter growth rate) with an annual rate of 5.7 ± 0.7 mm*yr −1. At Diguet the L. margaritae growth rate at 5m was 3.4 + 0.5mm*yr−1. Annual diameter growth rates for S. trichotomum were 7.7 ± 1.1 and 9.9 ± 0.8 mm*yr−1 at Requeson and Diguet, respectively. Growth rates increased with rhodolith size (range 2–10 mm*yr−1 for individuals 16–65mm diameter). Low growth rates and predicted ages of these larger individuals ranging from 11–17 years old, suggest slow population recovery times.; Species richness and abundance were significantly higher in rhodolith relative to sand habitat in Bahia Concepcion. This pattern is largely driven by cryptofaunal densities with cryptofaunal abundance increasing with rhodolith complexity and available space. In addition, I found the abundance of the catarina scallop (Argopecten ventricosus) was 12 times greater in rhodolith beds vs. sand flats. In a series of field and laboratory experiments conducted to identify the life-history stage where this pattern was established, I found that larval delivery and substrate-independent recruitment did not differ between sites. However, site-independent recruitment to rhodolith substrates was 26 times higher than that of sand, and post-settlement mortality and movement reinforces this pattern. Chemical and structural features of rhodoliths induce larval settlement and promote higher post-larval growth. Thus, adult distribution patterns are established through larval settlement preferences for complex coralline substrates and maintained through processes working at later life stages. Overall, these results suggest that preservation of scallop and associated populations is linked to protection of rhodoliths from anthropogenic disturbances.