ALBERT

Notes: Abstract We studied the effects of grazing by two species of sea urchins on two species of kelp (Macrocystis pyrifera and Pterygophora californica) in the San Onofre kelp bed in southern California from 1978 through 1981. Both red sea urchins, Strongylocentrotus franciscanus, and white sea urchins, Lytechinus anamesus, were abundant and lived in aggregations. The purple sea urchin (S. purpuratus) was rare at the study site and was not studied. The aggregations of red urchins were either relatively small and stationary (for over 3 yr) or relatively large and motile (advancing at about 2 m mo−1). Both stationary and moving aggregations were observed at the same time, and within 100 m of one another. Stationary aggregations of red urchins probably subsisted mainly on drift kelp and had no effect on kelp recruitment or on adult kelp abundance. In contrast, red sea urchins in large, motile aggregations or “fronts” ate almost all the macroalgae in their path. The condition of their gonalds indicated that red urchins in fronts were starved relative to red urchins in the small, stationary aggregations. Large, motile aggregations developed after 2 yr of declining kelp abundance (probably due largely to storms). We propose that a scarcity of drift algae for food results in a change in the behavior pattern of the red urchins and thus leads to the formation of large, motile aggregations. The aggregations of white urchins, which occurred along the offshore margin of the kelp bed, were large, but relatively stationary. The white urchins rarely ate adult kelps, but grazed extensively on early developmental stages of kelps and evidently prevented seaward expansion of the bed. The spatial distribution of both types of red urchin aggregations appeared to be unrelated to predation pressure from fishes or lobsters.

Notes: Abstract Instantaneous relative growth rates, μ (d-1), were measured for juveniles of the giant kelp Macrocystis pyrifera transplanted to study sites in Southern California kelp forests between 1978 and 1982. Growth rates ranged from negative values (indicating loss of tissue) to 0.03 (doubling of total frond length every 19 d). Multiple regression analysis of growth versus irradiation, temperature, nitrogen concentration and amount of fouling revealed that all these factors had significant effects, together accounting for about 50% of the total variance. Elevated irradiation and nitrogen levels had strongly stimulatory effects (tissue nitrogen may have been more critical than ambient nitrogen for growth), while high temperature and fouling had strongly inhibitory effects. Irradiation was the most important factor influencing growth in 6 of the 8 transplant experiments. During these 6 experiments, the compensating irradiation level (below which there was no growth) was between 0.4 and 0.7 E m-2 d-1, and saturating irradiation was between 2 and 3 E m-2 d-1. During two of the experiments, growth was apparently limited by extremely high temperatures or low nitrogen levels. Quantum irradiation levels in the kelp forest were generally between the compensation and saturation levels. However, irradiation levels occasionally dropped below the compensation point for several months. Irradiation was occasionally low enough to limit the distribution of juvenile kelp by inhibiting growth, especially in the deeper portions of the kelp forest and under dense canopies formed by adult plants.

Notes: Abstract Laboratory experiments were conducted in 1984 to test for an interaction between irradiance and temperature in controlling sporophyte production in the giant kelp Macrocystis pyrifera. Gametophytes from plants in the San Onofre kelp forest were cultured under a combination of seven irradiance (2.9 to 30 μE m-2 s-1) and ten temperature (11° to 20°C) levels. The minimum daily quantum dose for fertility of female gametophytes during the 42 d culture-period was 0.25 E m-2 d-1. The saturation quantum dose was approximately 0.60 E m-2 d-1. Temperatures between 11° and 20°C had little effect on the fertilityirradiance relationship, except at irradiations near the threshold and saturation levels. Cultures at 0.60 E m-2 d-1 had significantly lower fertility after 6 wk at 20°C than at the lower temperatures and there was a trend toward lower fertility at temperatures greater than 15°C in cultures at 0.25 E m-2 d-1. These differences were also reflected in the development time for female gametophytes. The 11°C cultures reached 50% fertility slightly faster and the 20°C cultures slower than cultures at intermediate temperatures.

Notes: Abstract The relative growth rates of juvenile Macrocystis pyrifera in southern California kelp forests were substantially reduced during the El Niño of 1982–1984. The lower growth rates were correlated with increased temperature and decreased nitrogen availability. Fertilization of juvenile plants with slow-release nitrogen-phosphorus fertilizer increased their growth rates to levels previously observed when temperatures were low and nutrient levels were high. The limitation in growth of M. pyrifera by levels of available nutrients during El Niño was in contrast to the usual limitation of growth by irradiance during non-El Niño years. Thus, there was a shift in the relative importance of factors controlling growth of juvenile M. pyrifera during El Niño.

Notes: Summary Experiments were performed to determine if earlier colonists inhibited, enhanced, or were necessary for establishment of later colonists during development of an estuarine fouling community at Lewes, Delaware. We determined the significance of earlier stages on the successional process by functionally removing early colonizing species. Since settlement of sessile invertebrates onto our experimental test plates was seasonal, we were able to accomplish functional removal of early colonists by putting out clean test panels after these species had ceased settling. Comparisons between panels initially submerged at three different times in 1974 and 1975, and between panels put out at one-month intervals throughout the study (to describe seasonal settlement patterns) allowed us to determine interactions between adult populations of earlier colonists and colonizing individuals of later arriving species. The dominant sessile species in our system and their times of settlement were: a barnacle (Balanus improvisus) — April through June, a polychaete (Hydroides dianthus) — July and August, a tunicate (Molgula manhatensis) — June through October, a hydroid (Tubularia crocea) — July through October, and a mussel (Mytilus edulis) — November through April. All successional series eventually came to be dominated by M. edulis, and it persisted as the dominant for over a year. A variety of species interactions were observed. M. edulis inhibited colonization by all other dominants and B. improvisus partially inhibited settlement of M. manhattensis. The presence of adult M. manhattensis had no influence on summer settlement of T. crocea, but the hydroids enhanced settlement of tunicates in the fall. During both years of our study, larger settlements of mussels were noted on panels harboring tunicates and hydroids than on bare surfaces. H. dianthus, on the other hand, became established only on bare substrates, and colonization was almost totally inhibited by other dominants. Development in our fouling community did not conform to any single model of community development presented to date. Instead, components of several models were observed within our relatively simple (in terms of number of species) system. For example, facilitation (enhancement of later colonists by earlier ones) and inhibition (resistance of earlier colonists to invasion by later colonists) were both observed. However, we found no evidence earlier colonists were essential for establishment of the next developmental stage. In fact, inhibitory interactions appeared to be much more prevalent than facilitative interactions. The former may also have more profound effects on community development since they more often determine eventual species compositions.

Notes: Summary The bunchgrass growth form, which is very prominent in water-limited environments, can result in considerable self-shading of photosynthetically active foliage. The consequences of this growth form for light interception and water-use efficiency (photosynthesis/transpiration, P/T) were investigated for two Agropyron species which differ in tussock density and degree of self-shading. During the period of most active gas exchange, the tussocks were very compact and photosynthesis of shaded foliage was markedly light-limited. Stomatal control of older shaded foliage was poorly attuned for water-use efficiency. At low light, P/T decreased and intercellular CO2 concentrations increased. Despite differences in architecture and amount of shaded foliage, P/T of whole tussocks under ambient field conditions did not differ between these species. Partial defoliation decreased, rather than increased, P/T, primarily as a result of the poor photosynthetic light harvesting by the remaining foliage. Despite self-shading, the architecture of widely-spaced bunchgrasses provides for interception of as much direct beam solar radiation as is calculated for a rhizomatous grass occupying an area six-fold greater than the ground area underneath the canopy of these bunchgrasses.