Light penetration in a temperate meso-tidal lagoon: Implications for seagrass growth and dredging in Tauranga Harbour, New Zealand

Benthic plants such as seagrasses rely on light availability, which is controlled by light attenuation in the water column. Elevated suspended sediment within estuaries strongly influences light attenuation and is governed by the recent history of natural events and human activities. To determine the constituents controlling light penetration (quantified by the irradiance attenuation coefficient, Kd(PAR)), we measured irradiance profiles, suspended particulate matter, chlorophyll-a and coloured dissolved organic matter (CDOM) in a barrier-enclosed estuary in New Zealand, subjected to dredging activity. These discrete measurements were then used to relate a continuous records of turbidity to potential light availability at multiple sites within the estuary. To do this, we use a regression model to enable turbidity measurements to be used as a proxy to estimate Kd(PAR). Measured Kd(PAR) ranged from 0.16 m-1 to 0.98 m-1 with overall average of 0.40 m-1, while Kd(PAR) inferred from continuous turbidity (which included many more events) reached higher values with average of 0.63 m-1. Continuous measurements of turbidity taken around the harbour dredging sites during dredging were slightly higher at some sites and slightly lower at others (∼2 mg l−1) compared to background measurements, indicating dredging had no consistent effect on conditions. Variations in Kd(PAR) were explained mostly by suspended particulate matter (accounting for 38%–50% of variance), chlorophyll-a (explaining 25%–33%) and to a lesser extent by coloured dissolved organic matter (CDOM explaining 10%–28%). Inferred benthic light availability in the intertidal zone when immersed ranged from 28% to 76% of surface irradiance, suggesting that light availability in the intertidal zone is not limiting seagrass photosynthesis but that growth in subtidal areas such as channels may be impacted. Results, inferred from continuous turbidity measurements, demonstrate the critical importance of including storm events in monitoring to develop robust limits on light attenuation for management.