Figure 1.
Halophila ovalis (A), Halodule uninervis (B), Zostera muelleri in the experimental units after 3 weeks exposure to 9 PSU (C) and a Zostera muelleri meadow in Gladstone Harbour, Australia where experimental plants were collected (D).
Figure 2.
Experimental set-up showing three chilled water baths each with four randomly allocated sumps immersed within them.
Each sump contained one of the 12 salinity treatments. Water was piped from the sump to the four replicate tanks and back again on closed-circulation.
Table 1.
Results of single factor repeated measures analysis of variance (RM-ANOVA) for change in shoot density at salinity treatments of 3 to 36 PSU in three seagrass species: Halophila ovalis, Halodule uninervis and Zostera muelleri.
Figure 3.
Change in Halophila ovalis shoot density relative to pre-treatment (week 0) (y-axis) as indicated by colour shading from 100% loss (red) through to 400% increase (blue), at salinities 3 to 36 PSU (x-axis) after 1 through to 10 weeks of exposure to treatment salinity (z-axis).
n = 4.
Table 2.
Summary of Tukeys Post-hoc comparisons for each week for change in shoot density.
Figure 4.
Change in Halodule uninervis shoot density relative to pre-treatment (week 0) (y-axis) as indicated by colour shading from 100% loss (red) through to 400% increase (blue), at salinities 3 to 36 PSU (x-axis) after 1 through to 10 weeks of exposure to treatment salinity (z-axis).
n = 4.
Figure 5.
Change in Zostera muelleri shoot density relative to pre-treatment (week 0) (y-axis) as indicated by colour shading from 100% loss (red) through to 400% increase (blue), at salinities 3 to 36 PSU (x-axis) after 1 through to 10 weeks of exposure to treatment salinity (z-axis).
n = 4.
Figure 6.
Foliar surface area (SA, cm2) calculated from shoot density, leaves per shoot and leaf length and width of H. ovalis (A), H. uninervis (B) and Z. muelleri (C) after 10 weeks at treatment salinity.
n = 4 ± SE
Figure 7.
Sexual reproduction in Halophila ovalis under salinity treatment conditions showing (A) reproductive potential which is the highest mean (total number of flowers and fruits) recorded for each treatment in weeks 6–10; and, (B) reproductive output (total number of flowers and fruits) correlated with shoot density at 10 weeks.
n = 4 ± SE.
Figure 8.
Leaf extension rate (mm d−1) for H. uninervis (A) and Z. muelleri (B) after 10 weeks exposure to hypo-salinity.
n = 4 ± SE.
Figure 9.
Conceptual summary of the seagrass responses to hypo-salinity.
High (marine, 36 PSU) salinities are “optimum”, as shoot density steadily increased throughout the experimental period at this salinity while sexual reproduction (for H. ovalis) was at its “peak”. At slightly depressed salinities (30–33 PSU) there appeared to be a “sub-optimal” transition zone as shoot density showed minimal increase and, furthermore, sexual reproduction (for H. ovalis) was low. With further hypo-salinity (<30 PSU), a stress-induced morphometric response was associated with a re-prioritisation of resources that saw massively increased shoot density (and leaf area) and reduced sexual reproduction. At extreme hypo-salinity (3–6 PSU) plant mortality occurred. The cut-off for each response phase moved to higher salinities with increased duration of exposure.
Table 3.
Summary of the sub-lethal (e.g. growth and photosynthesis or some shoot/biomass loss), and complete mortality thresholds (PSU) for seagrasses in responses to hypo- and hyper-salinity exposure.