Aspects of the energetics of the cockle Chione (Austrovenus) stutchburyi in the Avon-Heathcote Estuary, Christchurch, New Zealand.

Aspects of the energetics of the cockle Chione (Austrovenus) stutchburyi in the Avon-Heathcote Estuary, Christchurch, New Zealand. R.L. Stephenson. Department of Zoology, University of Canterbury, Christchurch, New Zealand. 1981. Aspects of the energy flow through Chione (Austrovenus) stutchburyi (Wood, 1828), a major member of the benthic macrofaunal community of the Avon-Heathcote Estuary, are investigated in order to define the role of this species and to test the hypothesis that this animal has a major role in the energy flow of the entire system. An historical account of the small (6 km2 area), shallow, mainly intertidal, bar-built, Avon-Heathcote Estuary (44°33'S, 172°44'E) documents the alteration in physical and biological characteristics with the expansion of the city of Christchurch within its drainage basin. In an introduction to the cockle, the nomenclature is discussed, and a description of larval development of C. stutchburyi reared in the laboratory is presented. The distribution of C. stutchburyi in the Avon-Heathcote Estuary was analysed from a survey of 200 sites. Relationships between size, weight and age, based upon 1000 individuals, allowed estimates of biomass and production; and their spatial distribution. Distribution is shown to be limited to levels on the shore with greater than 1.5 hours of water cover per tide and to sediments containing less than 50% mud and a mean sand diameter smaller than 2.25 ø. Preference of C. stutchburyi for greater than nine hours of water cover per tide, 10-30% mud content of the sediment, 2.5 - 3.0 ø mean sand diameter and 2.7 ø median sand diameter was assessed by analysis of the distribution of biomass and density. Density manipulation field experiments were conducted in an attempt to define the magnitude and distribution of recruitment, and to investigate the response of adult populations to disturbance and changes in density. Energy budget calculations for C. stutchburyi show a total winter biomass of 8.2 x 107 to 1.7 x 109 g in the Avon-Heathcote Estuary (energy content of 1.62 x 109 to 3.4 X 1010 kJ). On an areal basis, energy assimilated is estimated to be 4.3 x 102 to 4.7 X 103 kJ m-2 yr-1 ; between respiration (96.7 - 2.0 x 103 kJ m- 2 yr-1 ) and total production (3.3 x 102 - 2.7 X 103 kJ m-2 yr-1). Total production is partitioned into production of body growth (32.2 - 2.0 x 102 kJ m-2 yr-1), production of reproductive products (1.1 x 102 - 2.3 X 103 kJ m-2 yr-1) and mortality (2.0 x 102 kJ m-2 yr-1). A stable carbon isotope study of C. stutchburyi and possible food sources revealed a range in б13 C values of cockle tissue in parts of the population separated by less than two km, but subject to different hydrological regimes: This is clearly attributable to differences in the marine and terrestrial contributions of carbon to the suspended particle load (and therefore the diet), and proves that C. Stutchburyi utilises allochthonous organic carbon from the drainage basin, as well as marine carbon. Energy flow through the entire estuary (calculated by the energy analysis methods of H.T. Odum) is estimated to be approximately 5.2 x 106 kJ m-2 yr-1. The physical energy in flows of freshwater and the tide is the most significant contribution, followed by nutrient energy input from the Bromley Oxidation Ponds and the chemical energy of seawater dilution by freshwater. Two models of energy flow through C. stutchburyi and other major compartments of the Avon-Heathcote Estuary are presented and used to simulate the removal of oxidation pond discharge. From these models it is concluded that a significant proportion of the cockle population may be supported by the energy in pond discharge. Although the total energy flow through the C. stutchburyi population is small in comparison to that of the whole estuary (5.2 x 10-2%), the cockle is concluded to be especially important to the energetics of the entire estuary because of the high connectivity with other components of the system, and its intimate association with the particulate organic matter food chain