Alfred W. Ebeling

Severe storm disturbances and reversal of community structure in a southern California kelp forest

Regular observations made over a period of 5 yr in four permanent transects provided data on plant, sea urchin, and fish densities which indicate that two unusually severe winter storms in 1980 (“Storm I”) and 1983 (“Storm II”) had different effects on a southern California kelp-forest community. Storm I removed all canopies of the giant kelp Macrocystis pyrifera, but spared most understory kelps, mainly Pterygophora californica. Hence, the previously large accumulation of detached drift kelp, mostly M. pyrifera, disappeared. Denied their preferred diet of drift kelp, the sea urchins Strongylocentrotus franciscanus and S. purpuratus then emerged from shelters to find alternative food. Without effective predators, they consumed most living plants, including the surviving understory kelps. This weakened the important detritus-based food chain, as indicated indirectly by declining abundances of algal turf and fish (Embiotocidae) that eat small animals living in turf. In 1983, Storm II reversed the process by eliminating exposed urchins, while clearing rock surfaces for widespread kelp settlement and growth. By summer 1984, the kelp grew to maturity to form extensive canopies despite elevated water temperatures during summer and fall of 1983. Thus, severe storms may have vastly different effects on community structure, depending on the state of the community before the disturbance.

Variation in Algal Dispersal and Recruitment: The Importance of Episodic Events

Spatial and temporal variation in dispersal and recruitment of the kelps Macrocystis pyrifera and Pterygophora californica and of filamentous brown algae (FBA) consisting primarily of Ectocarpus siliculosus were examined off southern California, USA. These three taxa of sublittoral brown algae show enormous differences in adult morphology and demography, but release propagules near the bottom that are similar in size, shape, and motility. Their weekly variation in recruitment was measured on replicate frosted glass slides placed at different distances from stands of fertile adults at three sites. For all taxa, significant temporal variation was observed in recruitment at the zoospore source as well as at distances out to 4000 m from the source. For the kelps, this variation was due to variable rates of both zoospore settlement and early postsettlement mortality. In general, recruitment density of both kelps rapidly declined with distance from the adult stand; significantly lower recruitment was observed at as little as 3 m away. In contrast, dispersal of FBA was much greater; no significant decrease in recruitment density was observed out to 500 m. The differences in dispersal between these two algal groups may result from behavioral differences of their zoospores. Unlike those of kelps, zoospores of FBA are positively phototatic. This enables FBA zoospores to remain in the water column longer, thus increasing the distances over which they disperse. The predominant pattern of limited dispersal in the two kelps, however, was not constant over time. Dispersal was greatly expanded during episodic periods of high recruitment that coincided with winter storms. Turbulent water flower associated with these storms may resuspend kelp zoospores normally occurring near the bottom and allow them to be transported greater distances by prevailing currents. Although species characteristics of the adult stand appeared to influence the distribution of recruits, the effect disappeared within 10 m of the stand. At a short—range study site, decreases in spatial variation in recruitment density between the zoospore source and 3 and 10 m away indicated that the distribution of zoospores became more uniform with distance. Likewise at a long—range site, recruitment out to 4000 m appeared to occur uniformly and coincided with recruitment at the zoospore source and at all intermediate distances. This uniformity and coincidence of recruitment indicated that dispersal over longer distances was probably via individual zoospores rather than via clumps of drifting plants or reproductive fragments as suggested in previous studies. Although episodic, such dispersal is probably essential in promoting the persistence of these plants in highly disturbed and unpredictable environments.

Community Recovery After Storm Damage: A Case of Facilitation in Primary Succession

Manipulations of early arriving, fast-growing algal stands, which appeared soon after a severe storm denuded a Southern California marine reef habiatat, indicated that the dense cover protected newly settled kelp plants from excessive damage by grazing fishes. This is an example of refuge facilitation in primary succession after a major natural disturbance, a mechanism that may contribute substantially to the regeneration of a kelp forest.

Dispersal in Kelps: Factors Affecting Spore Swimming and Competency

The distance over which propagules can successfully colonize new sites de- pends on processes that increase the time they remain competent while being dispersed. As do feeding larvae, algal spores can contribute to their own nutrition (via photosynthesis) during dispersal. We explored the dispersal potential of the kelps Macrocystis pyrifera and Ptervgophora californica in laboratory experiments by examining (1) how long their spores can swim, (2) the contribution of energy derived from photosynthesis to spore swimming duration, and (3) the ability of spores to germinate and attach after they stop swimming. Results indicate that under photosynthetically saturating irradiance no spores of either species can swim longer than 120 h; < 10% of the spores were still swimming after 72 h. When placed in the dark, spores did not swim longer than 72 h; < 10% remained swimming after 48 h. More importantly, spores did not die after they stopped swimming; most germinated in the water column and retained their capacity to produce viable sporophyte recruits. The ability of spores to attach after they stopped swimming differed between the two species; settlement density declined in Macrocystis and increased in Pterygophora. Thus, the viable planktonic stage of these algae is not necessarily restricted to the spore but may include later life history stages. These results provide biological evidence that the spores and germlings of these kelps can remain competent in the plankton for extended periods of time, which is consistent with our previous findings that their dispersal can occur over greater distances than previously thought possible.

The mating call of the plainfin midshipman fish, Porichthys notatus

A previously unreported kind of fish sound, which was produced spontaneously by nesting male Porichthys notatus, was recorded in the field and laboratory. Unlike other fish sounds, this monotonous ‘hum’ continues uninterrupted from a few seconds to over 60 min with a mean length of about 11 min. It has a fundamental frequency between 98–108 Hz. The hum was produced only at night. The hum stimulates male-searching behavior in gravid females and serves as an underwater acoustic beacon for mate localization in this nocturnally active species. Gravid females responded in identical manner to pure tones as to the hum and became most highly excited at ~95 Hz, but also responed actively to pure tones over a range of 85–115 Hz. Spent females, juveniles and ripe males showed little, if any, response to the same pure tones. Gravid females tracked in an 8 m diameter concrete tank oriented to the 95 Hz pure tone by swimming directly 2–3 m to the suspended speaker, or by swimming in a circuitous path to the source, then pausing, turning or stopping under the speaker or swimming up to and butting and nipping the speaker.

The influence of plant cover on surfperch abundance at an offshore temperate reef

SynopsisAt an offshore reef near Santa Barbara, southern California, young-of-year (‘young’) of five surfperch species (Embiotocidae: Embiotoca jacksoni, E. lateralis, Hypsurus caryi, Rhacochilus toxotes, Damalichthys vacca) once thrived in a dense kelp understory of Pterygophora californica and Laminaria farlowii, but disappeared after a severe storm in February, 1980 deforested their habitat. Measurements of fish density and kelp cover made before deforestation indicated that the young surfperch appeared in the spring and disappeared in the fall as cover increased and declined. Adult surfperch and large kelp bass (Paralabrax clathratus), which can eat young but not adult surfperch, remained all year. We tested to see if kelp cover was an essential refuge for the young by pruning back (‘thinning’) kelp blades from one of two transects. The thinning caused a significant redistribution of young which clearly avoided open spaces, but not of adults which are less vulnerable to predation. Measurements of fish density made after the storm-induced deforestation showed that adult surfperch and kelp bass still remained abundant even after the young surfperch were gone. Only after an abrupt reforestation in 1983, more than a year after the present study was terminated, were young once again seen on the reef. Although young surfperch may seek tiny prey living on kelp blades, most lines of evidence indicated that the distribution of the young is more a response to risk of predation. Hence the extent of kelp understory was probably the main determinant of the survival of young surfperch on the reef.

THE ROLE OF REPRODUCTIVE SYNCHRONY IN THE COLONIZATION POTENTIAL OF KELP

Dispersal is a key element in the recovery of populations that have been locally destroyed by disturbance. Surprisingly, many sessile species that seemingly have limited potential to disperse often rapidly colonize areas that have been recently disturbed. The synchronous release of propagules during periods that promote advection may extend the dispersal of such species and promote their rapid colonization. Alternatively, rapid recovery may result from colonization by dormant stages that survive disturbance. Here we test for reproductive synchrony as a way of extending the dispersal potential of two common seaweeds, the kelps Macrocystis pyrifera and Pterygophora californica. Synchrony in spore release in their case is likely to be particularly useful in extending colonization distance because fertilization in these species occurs after spore dispersal; synchrony in- creases the chance of fertilization by increasing the concentration of spores. We also evaluate experimentally the relative importance of dormant stages vs. recently settled spores in accounting for the rapid recovery of local kelp populations following severe disturbances. Reproductive synchrony was evaluated by following weekly changes in the reproductive condition of adult kelp. The degree of reproductive synchrony in both Macrocystis and Pterygophora was significantly greater than -that expected under conditions of asynchronous reproduction. In Macrocystis, periods of synchronous spore production and release occurred sporadically over a 2-yr period. At least 75% of the sampled population exhibited the same directional change in reproductive condition in 38 of 82 sample periods. Episodes of sig- nificant spore release varied in duration from as little as 1 wk to as much as 2 mo. The sharpest decline in reproductive condition occurred during a severe storm. In contrast to Macrocystis, relatively well defined cycles of spore production, maturation, and release were observed in Pterygophora. More than one cycle was observed within a single repro- ductive season, and each cycle lasted -3-4 wk. The different patterns and degrees of synchrony observed between the two species likely reflect the degree to which their pro- duction of spores is influenced by environmental conditions; spore production in Macro- cystis is greatly influenced by fluctuations in seawater temperature and nutrients while spore production in Pterygophora is not. Results from field experiments comparing the recruitment of small plants among rocks placed in the kelp bed for varying lengths of time indicated that microscopic life stages of Macrocystis and Pterygophora have little capacity for dormancy, and that the vast majority of recruitment resulted from recently settled spores. These results contrasted with those observed for the annual brown alga Desmarestia ligulata which showed a dormancy period of several months. Our findings suggest that mechanisms such as reproductive synchrony that extend the distances over which kelp spores can effectively colonize are likely to play a critical role in the dynamics of kelp populations, which often fluctuate greatly in time due to disturbance.

ROLE OF SETTLEMENT DENSITY ON GAMETOPHYTE GROWTH AND REPRODUCTION IN THE KELPS PTERYGOPHORA CALIFORNICA AND MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Laboratory studies were used to examine how variation in the density of spore settlement influences gametophyte growth, reproduction, and subsequent sporophyte production in the kelps Pterygophora californica Ruprecht and Macrocystis pyrifera (L.) C. Ag. In still (non‐aerated) cultures, egg maturation in both species was delayed when spores were seeded at densities 300 · mm−2. Although the density at which this inhibition was first observed was similar for both species, the age at which their eggs matured was not. P. californica females reached sexual maturity an average of 4 days (or ∼ 30%) sooner than did M, pyrifera. As observed previously in field experiments, per capita sporophyte production was negatively density dependent for both species when seeded at spore densities of 10 · mm−2. Total sporophyte production (i.e. number · cm−2) for both species, however, was greatest at intermediate densities of spore settlement (∼ 50 spores · mm−2). In contrast, total sporophyte production by P. californica steadily increased with increasing spore density in aerated cultures; highest sporophyte density was observed on slides seeded at a density of 1000 spores · mm−2. Preliminary experiments with P. californica involving manipulation of aeration and nutrients indicate that inhibition of gametophyte growth and reproduction at higher densities of spore settlement in non‐aerated cultures was probably caused by nutrient limitation.

Experimental evaluations of substrate types as barriers to sea urchin (Strongylocentrotus spp.) movement

Descriptive evidence that sandy surfaces and rock ledges inhibit progress of grazing sea urchins prompted an experimental investigation of physical obstacles to urchin movement in a subtidal area of reef and kelp off southern California in 1980. In laboratory experiments, we found that both red (Strongylocentrotus franciscanus) and purple (S. purpuratus) urchins can negotiate sand using their oral spines, although purple urchins are slower and more hesitant to do so. In field experiments, we observed the fates of starved red urchins transported to replicate plots within stands of sand-or rock-based understory kelp (Pterygophora californica). Urchins in rock plots retreated to nearby crevices from where they ate attached kelp. After finding kelp blades, urchins soon disappeared from sand plots because individuals in small groups may have difficulty holding and eating attached kelps on unconsolidated surfaces. In another experiment, red and purple urchins reached kelp on a rock ledge by mounting an artificial ramp. We conclude that by using their tube feet, individuals of both species move best over flat, hard surfaces, although soft substrate may constitute a major barrier only to purple urchins. In the absence of effective predator control, urchins can surmount most sand or rock barriers when water motion subsides. Hence, their ability to coordinate spine movements to negotiate soft substrates may be an adaptation to invade kelp refuges during quiet periods if preferred drift food is unavailable.

Mouth size and predator strategy of midwater fishes

Abstract It has been suggested that bathypelagic fishes should be equipped to eat any large or small food item that they should chance to meet in their relatively impoverished environment. The derived hypothesis that bathypelagic species are trophically generalized was indirectly substantiated in two taxonomic groups of midwater fishes. Bathypelagic members tend to be larger and have larger mouths than their mesopelagic relatives, but still have a well-developed pharyngeal basket for retaining small items. Bathypelagic offshoots of the primarily mesopelagic group of lanternfishes seem to have acquired larger mouths at the presumed metabolic expense of having proportionately larger bodies. The deeper-living members of the primarily lower mesopelagic or bathypelagic family Melamphaidae are further specialized in that they have disproportionately larger mouths with minimum additional body size. This discussed relative to theoretical considerations of foraging strategy and metabolic economy.