Abstract
Temperature-Size Rule (TSR) is a phenotypic body size response of ectotherms to changing temperature. It is known from the laboratory studies, but seasonal patterns in the field were not studied so far. We examined the body size changes in time of rotifers inhabiting activated sludge. We hypothesize that temperature is the most influencing parameter in sludge environment, leading sludge rotifers to seasonally change their body size according to TSR, and that oxygen content also induces the size response. The presence of TSR in Lecane inermis rotifer was tested in a laboratory study with two temperature and two food-type treatments. The effect of interaction between temperature and food was significant; L. inermis followed TSR in one food type only. The seasonal variability in the body sizes of the rotifers L. inermis and Cephalodella gracilis was estimated by monthly sampling and analyzed by multiple regression, in relation to the sludge parameters selected as the most influential by multivariate analysis, and predicted to alter rotifer body size (temperature and oxygen). L. inermis varied significantly in size throughout the year, and this variability is explained by temperature as predicted by the TSR, but not by oxygen availability. C. gracilis also varied in size, though this variability was explained by both temperature and oxygen. We suggest that sludge age acts as a mortality factor in activated sludge. It may have a seasonal effect on the body size of L. inermis and modify a possible effect of oxygen. Activated sludge habitat is driven by both biological processes and human regulation, yet its resident organisms follow general evolutionary rule as they do in other biological systems. The interspecific response patterns differ, revealing the importance of taking species-specific properties into account. Our findings are applicable to sludge properties enhancement through optimizing the conditions for its biological component.
Highlights
The Temperature-Size Rule (TSR) proposes a relationship between body size response and changing temperatures, and it has been documented for approximately 80% of ectotherms (Atkinson 1994)
We found that two rotifer species living in activated sludge adjusted their body size seasonally in response to temperature in accordance with the Temperature-Size Rule and that one of them responded to oxygen changes, corroborating an important hypothesis that oxygen is the ultimate factor behind TSR
Temperature is most positively correlated with pH and negatively correlated with mixed liquor suspended solids (MLSS), while oxygen content is positively correlated with SVI (Table 3)
Summary
The Temperature-Size Rule (TSR) proposes a relationship between body size response and changing temperatures, and it has been documented for approximately 80% of ectotherms (Atkinson 1994) It states that organisms grow slower but for a longer time and achieve bigger size at lower temperatures, while the opposite is true at higher temperatures. This phenotypic, intraspecific response has a series of implications for performance and population growth because body size is the life-history trait most directly linked to the lifetime reproductive success (Kozłowski 1992; Roff 1992; Stearns 1992).
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