The effect of planktivorous fish on the vertical flux of polystyrene microplastics

Figures & data

Figure 1. The system comprised of two identical non-transparent vertical columns (a) and the light intensity (b), temperature (c) and oxygen concentration (d) gradients in the control (dotted line, mean  + 1 SD) and experimental treatments (solid line, mean  _ 1 SD).

Figure 2. Relative concentration (%) of large (a) and small (b) MPs in the water column in the control (filled box, mean + 1 SD) and experimental conditions (empty box, mean _ 1 SD) and the number of large (c) and small (d) MPs in sediment traps in the control (dotted line, mean _ 1 SD) and experimental conditions (solid line, mean + 1 SD).

Table I. The results of the analysis of deviance (Type II Wald chi-square tests for model-fixed effects) of the generalised linear mixed effects model (GLME) to test the effect of experimental condition (Tr), depth (Dp), MP size (Size), the Tr × Dp, Tr × Size, Dp × Size and Tr × Dp × Size interactions on the distribution in water column and sedimentation rate of (MPs). The χ² is Chi-squared statistic associated with a P-value (Pr(> χ²)). Statistically significant differences are marked in bold (df – degree of freedom).

Parameter	Factor; interaction	df	χ^2	Pr(> χ^2)
MP in water column	Tr	1	0.737	0.392
	Dp	6	74.275	< 0.001
	Size	1	0.023	0.894
	Tr × Dp	6	19.748	0.003
	Tr × Size	1	7.552	0.007
	Dp × Size	6	345.804	< 0.001
	Tr × Dp × Size	6	53.366	< 0.001
MP sedimentation rate	Tr	1	6.341	0.012
	Dp	6	480.070	< 0.001
	Size	1	7743.538	< 0.001
	Tr × Dp	6	22.096	< 0.001
	Tr × Size	1	171.093	< 0.001
	Dp × Size	6	285.011	< 0.001
	Tr × Dp × Size	6	7.487	0.023

Table II. The results of the two-way mixed ANOVA for trimmed means to test the effect of the presence of fish on the bacterial abundances with treatment (Tr) as between observations factor and depth (Dp) as within observations factors. Statistically significant differences are marked in bold (df – degree of freedom, Q – Q statistics, P _corr. – p-value corrected).

Parameter	Factor; interaction	df	Q	P corr.
Bacterial abundance	Tr	1	0.458	0.519
	Dp	1	10.873	0.008
	Tr × Dp	1	49.576	< 0.001

Table III. The results of the two-way mixed ANOVA for trimmed means to test the effect of the presence of fish on the DW distribution in the water column and the DW sedimentation rate with treatment (Tr) as between observation factor and depth (Dp) as within observation factors. Statistically significant differences are marked in bold (df – degree of freedom, Q – Q statistics, P _corr. – p-value corrected).

Parameter	Factor; interaction	df	Q	P corr.
DW in water column	Tr	1	26.052	< 0.001
	Dp	3	23.461	< 0.001
	Tr × Dp	3	37.166	< 0.001
DW sedimentation rate	Tr	1	19.508	0.004
	Dp	3	28.072	< 0.001
	Tr × Dp	3	15.314	0.003

Figure 3. Distribution (a) and sedimentation rate (b) of DW in the control (dotted line, mean _ 1 SD) and experimental treatment (solid line, mean + 1 SD) and the relative distribution of fish (mean ± 1 SD, %) before (c) and after (d) the feeding session.

Figure 4. Fish faecal pellet with a single large particle and a few small MPs extracted from the sediment trap in the experimental treatment photographed under the UV microscope (Nikon Eclipse E450) with 4 × magnification (a) and a single particle of MPs extracted from the water in the experimental treatment (b) and in control (c) photographed by the UV microscope (Nikon Eclipse E450) with 8 × magnification.

Data availability statement

The data presented in this study are available on request from the corresponding author.