Nutrient over-enrichment and light limitation of seagrass communities in the Indian River Lagoon, an urbanized subtropical estuary

Abstract

Historically, extensive seagrass meadows were common throughout the Indian River Lagoon (IRL) in east-central Florida, USA. Between 2011 and 2017, widespread catastrophic seagrass losses (~95%) occurred in the IRL following unprecedented harmful algal blooms (HABs), including persistent brown tides (Aureoumbra lagunensis). Little is known about how dissolved nutrients and chlorophyll a are related to light limitation or how biochemical factors, such as the elemental composition (C:N:P) and stable isotope signatures (δ¹³C, δ¹⁵N), of seagrasses within the IRL relate to coverage. Accordingly, we conducted a survey from 2013 to 2015 at 20 sites to better understand these relationships. Results showed a negative correlation between DIN and salinity, indicating freshwater inputs as a DIN source. Seawater N:P ratios and chlorophyll a concentrations were higher in the urbanized, poorly-flushed northern IRL segments. K_d values were higher in the wet season and often exceeded seagrass light requirements (0.8 m^-1) for restoration, demonstrating light limitation. Species distribution varied by location. Halodule wrightii was ubiquitous, whereas Syringodium filiforme was not found in the northernmost segments. Thalassia testudinum was only present in the two southernmost segments that had the lowest TDN and highest light availability (K_d). Blade %N and %P also frequently exceeded critical values of 1.8% and 0.2%, respectively, especially in the northern segments. Further, δ¹⁵N was positively correlated with ammonium, suggesting wastewater as a major N source. The δ¹³C values indicated a trend of increasing light limitation from south to north, which helps explain the recent catastrophic loss of seagrasses in the northern IRL. Overall, elemental composition reflected high N-availability and seagrass species distributions were relatable to spatial trends in N and light limitation. For effective restoration, resource managers must reduce N-loading to the IRL to diminish HABs and increase light availability. Regular biochemical monitoring of seagrass tissue should also be implemented during restoration efforts.

Keywords: Attenuation; Chlorophyll a; Eutrophication; Irradiance; Nitrogen; Phosphorus; Species distribution; Threshold.