Estuarine and Coastal Marine Science (1976) 4, 23 1-233

Observations of Algal Zonation Resulting from Competition”

Thomas Hruby” Department of Oceanography, University of Washington, Seattle, Wash. 98x95, U.S.A.

Received a5June 1975

In the coastal waters of the Pacific Northwest Region it was found that Iridaea cordata is excluded from areas below its normal distribution by a competitive interaction with more rapidly growing kelps, especially Laminaria saccharina. In areas cleared of kelps, I. cordata grew to cover 20% of the bottom from a natural settlement of spores.

Introduction

Interspecific competition, whether it be chemical or physical, is believed to be important in controlling natural algal distributions in littoral communities (den Hartog, 1968; Dayton, 1971); especially in the often seen sharp, lower boundaries of species growth (Connell, 1972;

Chapman, 1973). Russell & Fielding (1974) and Fletcher (1975) have recently shown that several algal species have an antagonistic effect on the growth of others in culture. However, most observations of competition in natural populations have been obtained in the testing of other ecological hypotheses and direct evidence showing that zonation results from competition is often lacking.

In the protected coastal waters of Washington State (U.S.A.) and southern British Columbia (Canada) beds of the economically important red alga, Iridaea corduta (Turner) Bory, are not found lower than IXFI.S m below Mean Lower Low Water (MLLW, the o level on U.S. Coast and Geodedic Survey Charts) even though Waaland (1973) found that transplants of this species will grow well to 5 m below MLLW, with optimal growth occurring at 3 m below MLLW. In natural conditions the beds of I. corduta are often found bounded by extensive beds of the kelp, Laminariu succhu~ina (L.) Lamour. The sharp zonation between these two species was thought to result from interspecific competition after it was observed in 1972 that young blades of I. cordutu were growing within the area of the kelp beds, in the absence of L. saccharina during the fall, winter and early spring. Contrary to what is found in European waters, the plant identified as L. saccharina in Pacific Northwest coastal waters is a summer annual. The effects of competition on I. cordatu

were tested in the I. cordata-L. saccharina complex found off Turn Point on San Juan Island in the San Juan Archipelago (Lat. 48”31’44”N., Long. 122’58’15”W).

Methods

A square plot of approximately 3 m2 was cleared of foliose brown algae on 14 April 1973

on the north side of Turn Point. In the previous year this area was in the middle of an

“Contribution No. 863 from the Department of Oceanography. ‘Present address: Department of Botany, The University, Glasgow G12 8QQ,

Scotland.

232 T. Hruby

L. succha~ina bed which exceeded 200 m2 and the boundary between the bed of I. cordata and the kelp was at a level of 1.0 m below MLLW. The boundaries of the weeded area were marked with large rocks and a foam plastic ball on nylon twine was anchored in the middle to help in its location when diving. The plot was weeded of all foliose brown algae at monthly intervals and the amount of bottom covered by I. corduta was estimated within the same five, 0.25 m2, square quadrats during the summer growing season. The cover estimates were made by drawing the outline of the plants on a plastic sheet containing a square grid, with the outline corresponding to the visual aspect of the plants as they lay under an aluminium frame divided into IO cm squares. The cover of the species in each of the 25

small squares was estimated on a linear scale of I-IO, corresponding to cover values of o-~oo~/~ and the values thus obtained were averaged to give the cover in the quadrat. The quadrats were positioned with reference to the float in the weeded area and controls were marked off in a similar pattern at a horizontal distance of I m from the cleared area. Because of kelp density, the cover estimates in the control quadrats in June and September were only approximations, made with the frame resting on two or three layers of fronds. Cover estimates were also made in five quadrats located in the I. cordata zone.

TABLE I. Area covered ( %) by I. cordata in replicate quadrats placed in the weeded and control plots in the Laminariu zone, at depths of 1.3-1’5 m below MLLW; and in the 1. corduta zone at 0.5 m below MLLW

Laminaria zone Zridaea zone Weeded plot Control plot

Quadrat number I 2 3 4 5 Mean I 2 3 4 5 Mean I 2 3 4 5 Mean

Date (1973) 14 April 610 5 8 2 (6) 5 7 3 10 2 (5) 12 9 26 29 8 (17) 5 May 8 20 14 14 4 (12) 6 6 3 9 2 (5) 12 June IO 38 32 I5 IO (21) approximation (2) 42 95 28 36 87 (58)

only 18 September 14 35 30 I5 9 (20) no I. corduta (0) 41 98 60 71 75 (71)

found

Results

The five weeded quadrats showed a marked, and statistically significant (P<o.og for May, June and September) increase in the growth of I. corduta when compared with the controls (Table I). In June the I. cordutu in the weeded plot became fertile at the same time as in its usual zone. Other species were found growing in the weeded plot which were not found in the control. Those identifiable underwater included Polyneura Zutissima (Harv.) Kylin, Neougurdhielhz baileyi (Kutz.) Wynne & Taylor, Rhodymeniu pulmutu (L.) Greville, and a species of Desmarestiu. However, by 13 June, I. cordutu plants had reached 0.3 m x0.6 m in size and were the dominant species in the weeded plot. R. pulmuta, which was second in dominance to I. cordutu, had a maximum size of approximately 0.2 m x 0.3 m and a mean cover of only 12%. The young I. cordutu and the sporelings of other species originally present in the control quadrats had disappeared by September and an additional 20 m2 of bottom under the kelps was searched without finding a single blade of I. corduta or any other foliose red alga.

The kelps had rapidly grown larger than the 1. cordata in the weeded plot even though their growth did not begin until the end of April. By June the kelp fronds had grown to be between I m and 2 m long, 0.4-0.6 m wide and were often overlapping themselves in layers

Algal zonation resulting from competition 233

up to three fronds deep. The kelps, which also included some plants of Costaria costata (Turner) Saunders and AZ&z marginata Postels & Ruprecht, disappeared in October leaving behind a barren substrate, which a month later was again covered with I. cordutu blades up to 5 cm long, growing down to a level of 4.0 m below MLLW.

Discussion

Since the only environmental change carried out in this experiment was the removal of the large kelps from a small part of their bed, the exclusion of I. cordutu from the deeper sub- strates can be considered a result of competition, The exclusion seems to be a dynamic process which occurs every year since I. cordutu was seen to re-establish a population in the ‘Luminuriu’ zone in two consecutive years. Thus the zonation line between the two species can vary depending on the condition limiting the growth of L. saccharina to the deeper zone.

The mechanism for the competitive exclusion observed has yet to be determined, but qualitative observations indicate that the causal factor is competition for light and may be similar in principle to the competitive exclusion of Succorhizu polyschides by Luminuriu

hyperboreu found by Norton & Burrows (1969). By early summer the kelps in their bed had grown to be much larger than the I. cordutu in the weeded plot, indicating that the kelps normally can overgrow this species in the ‘Luminuriu’ zone and thereby shade it. No part of the bottom in the kelp bed was visible below the layers of fronds, further indicating that there must have been a considerable reduction in the light reaching the substrate. Even at times when water motion causes the fronds to move, the necessary amount of light might have been kept from the bottom by the fronds forming the bottom layers.

This research was carried out in partial fulfilment of a Master of Science degree requirements at the University of Washington and was supported by funds provided through the Washing- ton Sea Grant Program as a part of the National Sea Grant Program which is maintained by the National Oceanic and Atmospheric Administration of the U.S. Department of Commerce. I wish to thank Dr J. R. Waaland, Dr J. Lewin and E. Duffield for their helpful discussions and support in the field.

References

Chapman, A. R. 0. 1973 A critique of prevailing attitudes toward the control of seaweed zonation on the sea shore. Botanica marina 16, 80-82.

Connell, J. H. 1972 Community interactions on marine rocky intertidal shores. Annual Revue of Ecology and Systematics 3, 169-192.

Dayton, P. K. 1971 Competition, disturbance and community organization: the provision and sub- sequent utilization of space in a rocky intertidal community. Ecological Monographs 41, 351-389.

den Hartog, C. 1968 The littoral environment of rocky shores as a border between the sea and the land and between the sea and the fresh water. Blumea x6,374-393.

Fletcher, R. L. 1975 Heteroantagonism observed in mixed algal cultures. Nature 253, 534-535. Norton, T. A. & Burrows, E. M. 1969 Studies on marine algae of the British Isles. 7. Saccorhiza

potyschides (Lightf.) Batt. British Phycotogicat Journat 4, 19-53. Russell, G. & Fielding, A. H. 1974 The competitive properties of marine algae in culture. Journal of

Ecology 62,689-698. Waaland, J. R. 1973 Experimental studies on the marine algae Irkfaea and Gigartina.Journal of Experi-

mental Marine Biology and Ecology II, 71-80.