I apologize for my delay in getting back to you all, but thank you for the thoughtful replies. In re-reading this post, I saw that I posted the same link twice. The study that references increased growth (therefore increased consumption and/or less daily energy expenditure) with sediment turbidity vs algal turbidity is here. Sorry about that.
https://www.utoledo.edu/nsm/lec/research/im/docs/cjfas-2013-0528.pdfIf I'm understanding the results correctly, it is a fairly well designed study that indirectly argued against conventional wisdom of pond fertilization to induce an algal bloom. Here are a few of their concluding points:
"Cohorts modeled with high sediment turbidity had the highest larval growth rate and underwent the ontogenetic shift 5 days earlier, on average, than cohorts modeled with low sediment turbidity (Fig. 4a). However, the larval growth advantage did not continue into the
juvenile phase when sediment turbidity remained high; the low sediment fish quickly surpassed the high-sediment fish in length.
Fish produced in low-sediment conditions were 25% larger than those produced in high-sediment conditions (82 versus 63 mm, Fig. 4a) at 124 days. Higher total growth, however, did not confer higher survival, as high-sediment conditions produced the lowest starvation mortality rate (Fig. 4b).
Fish in algal turbidity grew more slowly than those in sediment turbidity (Fig. 4a). Constant low-algae conditions produced fish that averaged 38 mm in length and that did not make the ontogenetic shift until approximately 100 days posthatch (Fig. 4a). Cohorts modeled with continuous high-algae conditions averaged only 24 mm in length, and these individuals generally never achieved the size associated with the ontogenetic shift (Fig. 4a). The algal cohorts also had significantly increased starvation mortality when compared with the sediment cohorts. Individuals in low-algae conditions experienced nearly 80% mortality, and those in high-algae conditions experienced over 99% mortality owing to starvation (Fig. 4b). Because survival was very poor in the high algae conditions, additional runs were completed to improve the sample size of survivors to estimate growth.
Using the relationships observed in the static-condition scenarios, we developed a “best growth” model that would represent growth of yellow perch under ideal turbidity conditions. The best growth model applied high sediment turbidity until the mean cohort length exceeded the threshold to switch to the juvenile feeding regime. Turbidity was then switched to low sediment to maximize juvenile-stage growth through the remaining days. No algal turbidity was applied to this group, as even low levels of algal turbidity reduced growth rates in both developmental stages. The cohorts in this model averaged 96 mm in length at 124 days posthatch, which was approximately 4% larger than the largest observed length (western basin, 1987, 92.4 mm, Fig. 3a).
Our algal turbidity scenarios, which simulated algal blooms lasting for 2, 4, and 8 weeks at the end of the season, produced
reductions in growth of 11%, 23%, and 45.5%, respectively, compared with the best-case scenario (Fig. 5a). Additionally, starvation
mortality increased from <10% in the best growth model to >60% in the 8-week algal bloom treatment (Fig. 5b). A 1-week algal bloom
during the second week of the larval stage produced results similar to a 2-week bloom during the juvenile phase, with a 10%
reduction in mean cohort length (Fig. 5a) and starvation mortality of 19% (Fig. 5b). An algal bloom during the larval phase also delayed the ontogenetic shift in these cohorts by approximately 12 days (Fig. 5a).
Our IBM showed that prolonged sediment turbidity after the ontogenetic shift had less severe effects than algal turbidity but still produced reductions in growth and increases in starvation mortality when compared with the best growth model. The 2-week
extension of high-sediment conditions produced only a 3% reduction in mean cohort length at 124 days posthatch (Fig. 6a) and a 2%
increase in starvation mortality (Fig. 6b). The 4-week extension of high-sediment conditions produced a 12.5% reduction in growth
(Fig. 6a) and a 22% increase in starvation mortality (Fig. 6b); these results are similar to those obtained with a 2-week algal bloom
late in the season (Figs. 5a and 5b). The 8-week sediment turbidity treatment resulted in a 26% reduction in mean cohort length
(Fig. 6a) and a 28% increase in starvation mortality (Fig. 6b)."
What I hadn't considered prior to diving into this topic, is that turbidity from one cause does not have the same impact as turbidity from another cause. In other words, algal turbidity vs sediment turbidity impact the environment in distinct ways.
It makes sense at face value to fertilize in order to feed the lowest elements of the food chain, therefore I was surprised to see that algal turbidity led to as much as 45.5% reduction in growth and mortality was above 60% when compared to their "best growth" model that started with high sediment turbidity then changed to low sediment turbidity, with ZERO algal turbidity. Even low levels of algal turbidity resulted in 80% mortality and high algal turbidity led to 99% mortality.
What isn't clear (pardon the pun) is how this translates to other species.
What led me to dive into this topic is evaluating the turbidity and staining of my pond water. My next quest may be to dive into how stained (but otherwise clear) waters impact the aquatic environment. The waters around me are the color of ice tea; clear but brown stained from tannins (predominantly from Eastern Hemlock I presume). Although Secchi disc readings are often one foot or less, brook trout thrive.
Thanks for sending the Secchi disc vs NTU charts. I looked for quite awhile without success.
