Estimating the Microphytobenthic Contribution to Ecosystem Net Community Production in a Gulf of Maine Estuary: Damariscotta River Estuary, Maine, USA

Abstract

Productivity in shallow coastal regions of the ocean on a per area basis can outweigh that of the open ocean due to high nutrient inputs from land. Within these shallow regions light can reach the sediment surface resulting in the growth of benthic algae, such as mat forming diatoms. Primary productivity in coastal environments can have a significant benthic component but can vary due to parameters such as light that influence the spatial and temporal distribution of biomass. Light is often the limiting factor for benthic primary producers and an understanding of the of the benthic PE (photosynthesis versus downwelling irradiance) relationship is necessary for quantifying the benthic contribution to ecosystem photosynthesis. In this thesis I used a combination of whole core ex situ flux incubations for the total oxygen exchange rate (TOE) and microsensor oxygen profiling for the diffusive oxygen exchange rate (DOE) to determine benthic PE curves. The PE relationship was determined for the benthic community in the Damariscotta River Estuary (DRE), a highly dynamic productive estuary in mid-coast Maine, USA. PE relationships were measured in sediments from different depths by sampling a transect across the estuary. Based on the natural range of light (6 to 63 μmol photons m-2 s-1 measured during sampling) at the sampling sites, the corresponding sediment total O2 exchange rate ranged from -40 mmol O2 m-2 d-1 in the dark to 53 mmol O2 m-2 d-1 at the highest light level. Rates across the transect were similar between sites except at the deepest site in the middle of the channel was lower. The average transect TOE photosynthetic capacity (Pmax) was 395 mmol O2 m-2 d-1, the average photosynthetic efficiency (α) was 3.9 mmol O2 m-2 d-1 (μmol photons m-2 s-1)-1 and the average respiration (R) was 70.8 mmol O2 m-2 d-1. Chlorophyll a sediment concentrations showed no variation between sites while microphytobenthos biomass varied across the transect with the deepest site having significantly less biomass. Combining the TOE PE parameters with measurements of light attenuation, and surface irradiance, the net community production (NCP) was estimated for the month of July 2019. Positive NCP was found in the two shallow sites on either side of the transect reflecting net autotrophy, while negative NCP was found in the deeper sites of the transect, reflecting net heterotrophy. This pattern across the transect is driven by higher light availability at the shallow sites and not differences in the PE relationship except for the middle of the channel site. Quantifying benthic photosynthesis is needed for determining the productivity of the whole estuary. This is an important question as the DRE hosts a lucrative shellfish aquaculture industry and understanding the benthic contribution to primary productivity in the estuary is necessary for understanding how much shellfish aquaculture the estuary can support and its effect on the local ecosystem.