The natural of coastal light pollution and its relationship with morphological variation in a larval fish around Wellington, New Zealand

Environmental variation is a critical factor which influences the health and structure of marine ecosystems, from the organismal to community levels. The lunar cycle is an important environmental cue for biological activity across many taxa, including reproduction, migration, and feeding. In coastal ecosystems, marine life may be threatened by changes to the nocturnal light regime caused by artificial light at night (ALAN). This research investigated the nocturnal light environment around the urban area of Wellington, New Zealand, as well as potential implications for development of a pelagic larval fish. First, I quantified the relationships between urban lighting, cloud coverage, and the lunar cycle on sky brightness with field observations. Then, I used remote sensing techniques to analyse the spatial distribution of sky brightness around the Wellington region over a long time series. Finally, I assessed the correlation between morphology of pre-settlement larvae of common triplefin (Forsterygion lapillum) and the probable nocturnal light environment they experienced during development.

I recorded nightly sky brightness at two locations and two times of night over one lunar month in order to model the relationships between urban lighting, cloud coverage, and the lunar phase as predictors of overall sky brightness. Modelling showed that proximity to urban lighting significantly increased sky brightness across spatial and temporal gradients. Cloud coverage further expanded the spatial impacts of urban lighting on sky brightness beyond the area that could be reached by direct sources of artificial light. The lunar cycle was only an important predictor of sky brightness late at night, when there was minimal input from urban lighting, suggesting that the lunar cycle was partially masked by skyglow.

I used satellite imagery to create a time series of images showing nightly skyglow across the Wellington region and analysed the time series to identify spatial patterns in long- and short- term trends in unnatural sky brightness. I developed a remote sensing index based on previously observed relationships between urban lighting, cloud coverage, and the lunar phase, and found that it was an accurate representation of nightly sky brightness. Time series clustering revealed a spatial gradient of increased sky brightness with proximity to the city centre for both long- term and short-term trends. Analysis of short-term (inter-annual) trends showed that urbanized skies, up to 30km offshore, exhibited more variability, suggesting that the lunar cycle is being masked more in those areas. The spatial extent of increased brightness over offshore waters demonstrates the magnitude of potential effects of this environmental change for coastal marine life.

Morphological analysis identified a gradient in body shape of larval F. lapillum associated with exposure to ALAN during development. Fish from a brighter environment had smaller caudal peduncles and heads, traits which may be associated with decreased fitness in pelagic larvae. Variation in the light environment was a better predictor of multivariate morphology than sampling region. Complex relationships between other environmental variables more closely linked to sampling region, such as wave exposure and water temperature, and nocturnal lighting could be affecting larval morphology through a number of direct (i.e., metabolic costs) and indirect (i.e., altered trophic interactions) mechanisms.