Plant and frond dynamics of the giant kelp,Macrocystis pyrifera,forming a fringing zone in the Falkland Islands

Fluctuations in plant and frond characteristics are described for Macrocystis pyrifera (L.) C. Agardh (Laminariales, Phaeophyta) forming a fringing zone in the Falkland Islands. Giant kelp plants were sampled along a transect in the austral autumn (May 1986) and late spring (December 1986) which, according to previous frond weight analysis, were the times when extremes in population parameters were expected. Plant density and holdfast wet weights were similar for both seasons, but plants had more fronds and the fronds weighed more in spring than in autumn. Consequently, in autumn the frond biomass (1·1 wet kg m^?2) and productivity (34·1 wet g m^?2 d^?1) were lower than in spring, when a biomass of 5·0 wet kg m^?2 and a productivity of 72·4 wet g m^?2 d^?1 were recorded. Production of new fronds and loss of old fronds were determined at monthly intervals between April 1986 and March 1987. New frond production rates followed fluctuations in the quantity of light and varied between 0·08 and 0·48 fronds per plant per day. Frond loss rates did not show a seasonal pattern and fluctuated between 0·05 and 0·42 fronds per plant per day. It is suggested that the Falkland Islands Macrocystis population is more stable than most other giant kelp beds at high latitudes, because of the absence of winter storms.     

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.     

EFFECTS OF THE ENCRUSTING BRYOZOAN,MEMBRANIPORA MEMBRANACEA,ON THE LOSS OF BLADES AND FRONDS BY THE GIANT KELP,MACROCYSTIS PYRIFERA (LAMINARIALES)1

Juvenile sporophytes of the giant kelp, Macrocystis pyrifera (L.) C. A. Agardh, were transplanted from local kelp beds to stations located various distances from the outfall from an electrical generating station that was known to cause an increase in the settlement of fouling organisms. Plants near the outfall became heavily fouled by the encrusting bryozoan, Membranipora membranacea (L.), and lost about one-third of their blades during the course of the experiment. Blade loss was significantly correlated with amount of fouling. To test the hypothesis that fouling causes blade loss, we paired fouled and unfouled plants of about the same age, overall length, and number of fronds and placed them at stations in nearby kelp beds and near the outfall. At the stations in the kelp beds, the fouled plants lost blades more rapidly than the unfouled controls. However, at the station near the outfall the “control” plants quickly became fouled so there was little difference in treatments and there was no significant difference in blade loss. Plants fouled by Membranipora suffered greater blade loss than clean plants probably because fouled blades are fragile and break off easily and because fish bite off chunks of blade while foraging on the attached bryozoans.     

TRANSLOCATION OF 14C IN MACROCYSTIS INTEGRIFOLIA (PHAEOPHYCEAE)1,2

Translocation patterns in the giant kelp, Macrocystis integrifolia Bory, were investigated in situ using ¹⁴C tracer; sources and sinks were identified. Export was first detected after 4 h of labeling; experiments were routinely 24 h continuous ¹⁴C application. Mature blades exported ¹⁴C to young blades on the same frond and on younger fronds, as well as to sporophylls and frond initials at the bases of the fronds. Blades <0.3 m from the apex imported and did not export; this distance did not change seasonally. In spring export from blades 0.3–1.25 m from the apex was exclusively upwards; older blades also exported downwards. In fall downward export began 0.5 m from the apex, and blades >2 m from the apex exported exclusively downwards. Carbon imported by frond initials, young fronds, and sporophylls in fall may partly be stored for growth in early spring. No translocation was seen in very young plants until one blade (secondary frond initial) bad been freed from the apical blade; this blade exported to the apical blade for a time, but imported when it began to develop into a frond. The second and third formed blades on the primary fronds (sporophylls also exported when <0.3 m from the apex, and later stopped. Frond initials and sporophylls on later-formed fronds did not export at all. The translocation pattern in M. integrifolia differs from that previously reported in M. pyrifera in seasonal change and in distances from the apex at which the changes take place.     

EFFECT OF HIGH IRRADIANCE ON RECRUITMENT OF THE GIANT KELP MACROCYSTIS (PHAEOPHYTA) IN SHALLOW WATER1

Laboratory and field experiments were done hi Still-water Cove, Carmel Bay, California, and Monterey Harbor, California, to determine the effect of photosynthetically active radiation (PAR) on the shallow (upper) limit of giant kelp, Macrocystis pyrifera (L.) C. Agardh. At shallow depths, M. pyrifera did not recruit or grow to macroscopic size from gametophytes or embryonic sporophytes transplanted to vertical buoy lines; sharp decreases in PAR with depth coincided with observed recruitment and sporophyte distributions. Shade manipulations indicated that settlement of M. pyrifera zoospores was decreased, but not prohibited, by high PAR. Postsettlement stages (gametophytes and embryonic sporophytes), however, survived only under shade. These results suggest that high PAR can inhibit the recruitment of M. pyrifera to shallow water by killing its postsettlement stages; whether or not ultraviolet (UV) radiation also inhibits recruitment was not tested. In either case, however, it appears that high irradiance (PAR and/or UV) regulates the shallow limit of M. pyrifera prior to temperature and desiccation stresses inherent to intertidal regions. In an additional experiment, recruitment or growth of transplanted gametophytes or embryonic sporophytes of Macrocystis integrifolia Bory also did not occur at shallow depths, suggesting that this shallow water species accesses high irradiance regions via a method other than sexual reproduction.     

Little evidence for climate effects on local-scale structure and dynamics of California kelp forest communities

Climatic variables such as temperature and precipitation play an important role in controlling local and regional scale differences in population dynamics and species distributions, and large-scale climatic events such as El Niño southern oscillation (ENSO) have been shown to affect population dynamics of key species in many ecosystems, particularly in kelp forests. Few studies have been able to evaluate the consequences of climate variables on the structure and dynamics of biological communities, in large part because the lack of data at appropriate spatial and temporal scales has made it difficult to adequately address local-scale responses of species and communities to such events over relevant time scales. Here, we combined an unprecedented dataset of kelp forest species' abundances from the Channel Islands, California with data for several local, regional, and global scale climatic variables to evaluate the temporal and spatial scale at which one can detect community-wide effects of climate variables, in particular ENSO events. We found large and significant local-scale differences in community structure, but these differences were not related to differences in climatic variables. Moreover, giant kelp abundance, which has been shown to be highly sensitive to water temperature and storm disturbance, was a poor predictor of community differences, and all communities tended to decline in abundance over the 20-year sampling period, suggesting a press perturbation to the system such as PDO cycles or sustained fishing pressure. Although ENSO events can have dramatic impacts on the abundance and distribution of giant kelp itself across the range of the species, such events appear to have little effect on local-scale kelp forest community structure or dynamics.     

EFFECTS OF SHADING BY ADULTS ON THE GROWTH OF BLADE-STAGE MACROCYSTIS PYRZFERA (PHAEOPHYTA) DURING AND AFTER THE 1982–1984 EL NIÑO1

The effect of shading by an adult canopy on blade-stage Macrocystis pyrifera (L.) C. A. Agardh was estimated by comparing the average growth rate of individuals under a canopy to that of individuals in a canopy gap. This comparison was made in 1983 during a strong El Niño and again in 1986 after the El Niño. Estimated nutrient concentrations in 1983 were two orders of magnitude below those in 1986, whereas ambient light levels were over 3 times higher. The kelp canopy caused similar proportional light reductions (20–30%) during both years. Blades grew 18% slower under the canopy than in the clearing in 1983 and about 77% slower under the canopy in 1986. Blade-stage individuals grew at the same rates in clearings in 1983 and 1986. Regardless of shading, the average growth rate of blade-stage kelp under the ambient, low-nutrient conditions of 1983 was higher than that later observed for multifronded juveniles during the same El Niño. The growth of blade-stage kelp was more like that of larger juveniles growing under high-nutrient conditions. The difference may be due to greater concentrations of nutrients very near the sea floor where single blades are growing compared to concentrations higher in the water column where larger kelp have most of their tissues.