APPELDOORN: VARIATION IN GROWTH RATE OF MYA ARENARIA 



regression indicated that growth was negatively 

 correlated with sedimentary hydrocarbons. 



DISCUSSION 



The observed relationship between latitude 

 and growth rate is not surprising, especially con- 

 sidering the range of temperatures reflected in 

 the data. Increasing growth would be expected 

 at higher temperatures owing to temperature's 

 direct effect on metabolism and length of the 

 growing season (Brousseau 1979). In addition, 

 with increasing temperature Mya is found lower 

 intertidally or even subtidally (Pfitzenmeyer 

 1972), thereby increasing its daily feeding period. 

 However, Belding (1930), Dow and Wallace 

 (1961), Newcombe and Kessler (1936), and Swan 

 (1952) have stated that local hydrologic and 

 edaphic conditions are more important than 

 temperature in affecting growth, and previous 

 studies have failed to quantify such a latitudinal 

 relationship. Newcombe (1936) and Turner 

 (1948) each noticed growth differences between 

 three populations which they attributed to tem- 

 perature. Brousseau (1979) showed a tendency 

 for Massachusetts populations to grow faster 

 than more northern ones, but the relationship 

 was not definite. Each of these studies suffered 

 from two deficiencies: Limited geographical 

 range and small number of sample sites. Under 

 these limitations, variations in growth rate due 

 to local conditions can mask any latitudinal 

 trends. The unaccounted variation (32%) in the 

 regression of logio(cu) on latitude is evidence for 

 this. 



Analysis of the latitudinal trend of growth rate 

 and its residual variation was facilitated by the 

 PCA results. The first component, northness, 

 correlated well with growth. Temperature and 

 tidal position had the highest loadings for this 

 component; their interrelationship and influence 

 on growth have already been discussed. As with 

 many factors, the components produced by PCA 

 represent an integration of effects and the corre- 

 lation between growth and northness may de- 

 pend upon factors other than temperature. 



Two other characteristics vary markedly with 

 increasing northness. The sediment becomes 

 coarser and more variable, and tidal range in- 

 creases. The tidal range increase, due to the 

 large tides of the Gulf of Maine and Bay of 

 Fundy, represents to some degree an increase in 

 tidal current. Belding (1930) considered current 

 the most important factor affecting growth. 



Coarser sediments are beneficial to growth by 

 allowing for ample water percolation, drainage, 

 and exchange (Dow and Wallace 1961; Swan 

 1952). The correlation of northness with coarser, 

 variable sediments reflects both their glacial 

 origins and the influence of current on their 

 deposition. 



Within northness, then, there are two sets of 

 opposing conditions which influence growth. 

 Temperature is positively associated with growth 

 while current and sediment characteristics are 

 negatively associated with growth. Since north- 

 ness is itself negatively correlated with growth it 

 must be concluded that the effects of tempera- 

 ture are overriding and dominant. 



The effect of siltiness on growth also represents 

 an integration of processes. In small quantities 

 silt and clay help stabilize surface sediments 

 (Kellogg 1905), but in large quantities they be- 

 come detrimental. Studies with Mya arenaria 

 have shown that excessive siltation can lead to 

 reduced feeding through clogging of the gills 

 (Belding 1930), to growth interruptions when silt 

 becomes trapped between the shell and mantle 

 (Shuster 1951), and to complete smothering and 

 death (Wilton and Wilton 1929; Dow and Wallace 

 1961). Silty sediments tend to be fairly consoli- 

 dated, and reduced growth has been observed in 

 such sediments (Swan 1952; Dow and Wallace 

 1961). High silt-clay is also indicative of a poor 

 current regime, itself a contributing factor to re- 

 duced growth. The negative correlation found 

 between siltiness and growth is, therefore, logi- 

 cal and consistent with previous reports. 



Sedimentary hydrocarbons, the third compo- 

 nent derived from PCA, were also negatively as- 

 sociated with growth. Many studies have shown 

 that the growth of Mya is adversely affected by 

 the presence of petroleum hydrocarbons (Dow 

 1975; Dow and Hurst 1975; Gilfillan and Vander- 

 meulen 1978; Gilfillan et al. 1976; Appeldoorn 

 1981). Hydrocarbon pollution can adversely af- 

 fect growth through direct toxicity, smothering, 

 and sediment compaction. 



CONCLUSIONS ON METHODOLOGY 



PCA produced meaningful, easily interpret- 

 able variables which led to results, when further 

 analyzed, that were lucid, rational, and consis- 

 tent with other studies. Since PCA will be limited 

 by the input data, sampling should be properly 

 designed or controlled. One advantage of PCA is 

 that variable integration effects can incorporate 



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