Population structure of a newly recorded (Halodule uninervis) and native seagrass (Halophila ovalis) species from an intertidal creek ecosystem

The present study documented the presence of seagrass Halodule uninervis for the first time along with previously documented Halophila ovalis at Haripur creek. The population structure of both these seagrass species is assessed. The physico-chemical parameters were similar for both seagrass species except for the sediment grain size fractions. The sand content of H. ovalis patches was 1.2-fold higher than H. uninervis beds, whereas the silt content of H. uninervis beds was 2-fold higher than H. ovalis patches. The pH levels were lower than the standard oceanic pH of 8.2. Macroalgae like Ceramium sp. and Gracilaria verrucosa were growing on the leaves of H. uninervis due to high nitrate and phosphate levels of the creek waters. Leaf reddening was only observed in the leaves of H. ovalis. Under similar environmental conditions, H. ovalis (5004 ± 114.51 ind. m-2) had a 2-fold lower shoot density than that of the H. uninervis (11598 ± 187.52 ind. m-2). Both above- and below-ground biomass of H. ovalis (96.34 ± 10.18 and 197.5 ± 18.30 g DW m-2) was 2-fold lower than that of H. uninervis (198 ±7.45 and 456 ± 9.59 g DW m-2). H. uninervis leaves were 9-fold longer than that of H. ovalis, whereas H. ovalis leaves were 5-fold wider than H. uninervis. The leaf plastochrone interval is 2.3 days for H. ovalis and 9.6 days for H. uninervis. Consequently, the leaf growth rate of H. ovalis is 2-fold lower than that of H. uninervis. H. ovalis had 2.6-fold longer internodes than H. uninervis. The root length of H. uninervis was longer than H. ovalis. Consequently, the shorter root length of H. ovalis led to higher branching frequency than H. uninervis. The total C and N content were higher in the leaves of H. ovalis than H. uninervis.

A Tripartite Interaction Between Spartina alterniflora, Fusarium palustre, and the Purple Marsh Crab (Sesarma reticulatum) Contributes to Sudden Vegetation Dieback of Salt Marshes in New England

Tripartite interactions are common and occur when one agent (an arthropod or pathogen) changes the host plant in a manner that alters the attack of the challenging agent. We examined herbivory from the purple marsh crab (Sesarma reticulatum) on Spartina alterniflora following exposure to drought or inoculation with Fusarium palustre in mecocosms in the greenhouse and in crab-infested creek banks along intertidal salt marshes. Initially, drought stress on S. alterniflora and disease from F. palustre were examined in the greenhouse. Then, a second challenger, the purple marsh crab, was introduced to determine how drought and disease from F. palustre affected the attraction and consumption of S. alterniflora. Plant height and shoot and root weights were reduced in plants subjected to severe drought treatment when compared with normally irrigated plants. When the drought treatment was combined with inoculation with F. palustre, plants were significantly more stunted and symptomatic, had less fresh weight, more diseased roots, and a greater number of Fusarium colonies growing from the roots (P < 0.001) than noninoculated plants. The effects were additive, and statistical interactions were not detected between drought and inoculation. Estimates of herbivory (number of grass blades cut or biomass consumption) by the purple marsh crab were significantly greater on drought-stressed, diseased plants than on healthy plants irrigated normally. Drought increased attraction to the purple marsh crab more than inoculation with F. palustre. However, when only mild drought conditions were imposed, plant consumption was greater on inoculated plants. Healthy, nonstressed transplants set into plots in crabinfested intertidal creek banks were grazed less each year than inoculated plants or plants that were exposed to drought. Several hypotheses relating to nutrition, chemotaxis, and visual attraction are presented to explain how stress from drought or disease might favor herbivory.