The phrase "as indicated by a gradual decrease" is correct and usable in written English.
You can use it when discussing trends or changes that are evidenced by a slow decline in data or observations.
Example: "The results of the study show a significant improvement in performance, as indicated by a gradual decrease in error rates over the past year."
Alternatives: "as shown by a steady decline" or "as evidenced by a slow reduction."
Successive addition of alumina caused a cumulative increase in water requirement as indicated by a gradual loss in workability.
After the initial uptake, at 40 min post-injection further uptake in both the liver and spleen for most liposome preparations appeared to be gradual over the 24 h as indicated by the gradual decrease in radioactivity taken from the heart (indicative of blood pool) which was similar in rate for all liposomes (Figs. 3 and 4).
It cause a gradual spread of plume P2 toward the western part of the Tokyo area, as indicated by a thick arrow in Fig. 9d.
This gradual increase may be associated with an increase in solar activity as indicated by an increase in sunspot numbers during 2010 and 2011.
The transmission blocking effect was dependent upon the antibody dose as revealed by a gradual decrease with increasing sera dilutions.
Flow cytometry analysis in HCT116 cells showed that TPEN toxicity is time-dependent as indicated by the gradual enrichment of the pre-G1 percentage in the cell population.
Another consequence of the passage of time, as indicated by fibrosis, is the gradual diminution of lymphoid infiltration (LI) which is mostly present in young tumours, especially those of high grades.
Our study showed that hAMCs decreased HSCs activation, as indicated by decreasing expression of a-SMA, GFAP and desmin.
The amount of hydrocarbon expelled from source rock increases at first followed by a gradual decrease as the depth or R o increases.
In the tilt group, the fluorescence intensity peaked when the swing tilted to the maximal angle, followed by a gradual decrease as the angle was reduced and a rapid increase when the angle was increased.
