Impacts of Episodic Freshwater Inflow Pulses on Seagrass Dynamics in the Lower Laguna Madre, Texas, 1998-2017

The Lower Laguna Madre of Texas (LLM) is a subtropical, hypersaline lagoon dominated by lush seagrass beds and characterized by a historically low freshwater inflow (FWI) regime. From 1998 to 2012, a net 8.8% seagrass decrease occurred in the LLM, primarily Syringodium filiforme and Thalassia testudinum, despite the concomitant expansion of Halodule wrightii. In the middle region of the LLM affected by a major FWI source, the Arroyo Colorado, the decrease was > 20%. We proposed that Thalassia and Syringodium decrease was largely due to high FWI pulses and N loading from an increasingly modified LLM watershed. Salinity modeling studies showed that FWI pulses lowered salinities for months in this region potentially associated with non-point source nutrient (viz. ungauged) inputs. At a site near the Arroyo Colorado, Thalassia biomass and shoot density were lower 319.3 g/m(2) and 567 shoots/m(2), respectively versus 1296 g/m(2) and 3382 shoots/m(2) for the most productive LLM site. Targeted sampling in four subwatersheds adjacent to the LLM demonstrated high dissolved N loadings (nitrate and organic nitrogen) to the lagoon. Nitrogen isotope analysis of LLM seagrass further indicated that nitrogen was likely from inland wastewater runoff from the Arroyo Colorado. We concluded that freshwater pulses to the lagoon produce hyposaline conditions and high nutrient loading, stressing seagrasses and resulting in diminished resilience and reduced distribution of Thalassia and Syringodium. With climate change, coastal population growth, and enhanced watershed drainage, episodes of hyposalinity, and enriched nutrient runoff are likely to increase in frequency and duration, altering seagrass resilience and diversity in the LLM.

Automated summary

The Lower Laguna Madre of Texas is a hypersaline lagoon with low freshwater inflow. The decrease in seagrass beds, particularly Thalassia testudinum and Syringodium filiforme, is attributed to high freshwater pulses and nutrient loading from the surrounding watershed. It is concluded that these factors stress seagrasses and diminish resilience, and with climate change and population growth, the frequency and duration of hyposalinity episodes and nutrient runoff are expected to increase, further altering seagrass diversity.