Seasonal growth dynamics of Ruppia maritima L.s.l. and Halodule wrightii aschers. In Southern Texas and evaluation of sediment fertility status

Annual growth dynamics of Ruppia maritima L.s.l. and Halodule wrightii Aschers. at two southern Texas, U.S.A., coastal sites were compared using plant biomass to monitor production. While sparse Ruppia normally coexists with dense Halodule in southern Texas bays, these two sites contained extensive stands of dense Ruppia mixed with or adjacent to Halodule. Corresponding measurements of water-column salinity and temperature revealed that vigorous Ruppia growth correlated positively with cool spring temperatures and not with low water salinities. In contrast, Halodule growth increased only after warm summer temperatures were reached. Partitioning of biomass into above-ground and below-ground tissues is suggested as an important autecological factor in resource competition between the two species. Ruppia, with a maximum 31% below-ground biomass in mid-spring, could be out-competed by Halodule, with a minimum of 66% below-ground biomass, when sediment or other conditions become favorable for Halodule growth.Analyses of sediment interstitial water from Ruppia or Halodule grass beds showed differences in rhizosphere nutrient pools characteristic of each species. Ruppia-dominated beds contained up to 15 μM nitrate during spring, whereas Halodule beds showed only a trace (< 1 μM). Halodule-dominated beds showed consistently higher levels of sediment H₂S compared to Ruppia areas (130 μM vs. 50 μM, respectively, average autumn concentration in top 10 cm). Based on these measurable edaphic differences, growth responses to sediment fertilization with two distinct fertilizer formulations were tested: (1) Osmocote^TM, an ammonium nitrate-based inorganic mixture, and (2) Hyponex^TM an organic mixture derived from fish meal. Contrasting responses to these two sediment fertilizers were observed. Halodule showed growth stimulation in early autumn by the organic fish meal only, while Ruppia responded to both types of fertilizer in both spring and early autumn. It is postulated that Ruppia normally grows on low-nutrient sediments, whereas Halodule prefers organic-rich sediments with substantial sulfate reduction activity.