**4. Conclusions**

A distinct salinity plume was observed around the outfall of Carlsbad Desalination Plant with a salinity increase of up to 2.7 units above ambient and extending 600 m offshore. This exceeds the maximum allowed salinity perturbation in the coastal zone as stated by the California Ocean Plan (2015 Amendment to Water Quality Control Plan). However, no significant impact was found on the benthic epifauna or infaunal composition and abundance in the same area, possibly due to an already disturbed conditions in this setting due to previous and ongoing discharge of post-cooling water from a local power plant. The coastal wave energy and mixing potential at Carlsbad Beach is high compared to other locations along the Southern and Central California coastline, and the lack of sufficient mixing of the brine at Carlsbad Beach raises concern regarding the efficiency of brine mixing at other proposed sites for desalination plants along the coast of California.

We recommend all future desalination facilities to be designed with optimal discharge options of either diffusor systems or a higher dilution of the brine prior to discharge. Furthermore, they should, where possible, be co-located with other water discharging facilities such as power plant or waste water treatment plants to restrict potential impacts to areas that are already disturbed from anthropogenic activities. Finally, we stress the need for long-term monitoring of coastal ecosystems in the area surrounding proposed and current desalination facilities to fully understand the ecological response to long-term exposure to salinity increase.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4441/11/2/208/s1, Figure S1: Benthic macrofauna. Figure S2: Sediment grain size distribution, Table S1: Water chemical properties. Table S2: Brittle star growth. Table S3: Wave energy and benthic habitat estimation.

**Author Contributions:** Conceptualization, K.L.P., D.P. and A.P.; Data Curation, K.L.P. and A.P.; Formal Analysis, K.L.P., N.H., B.G.R. and A.H.; Funding Acquisition, D.P. and A.P. and K.L.P.; Investigation, K.L.P., D.P. and A.P.; Methodology, N.H. and B.G.R.; Project Administration, A.P. and K.L.P.; Resources, A.P.; Software, N.H. and B.G.R.; Supervision, D.P. and A.P.; Validation, D.P.; Visualization, K.L.P., N.H. and B.G.R.; Writing—Original Draft, K.L.P.; Writing—Review and Editing, K.L.P., N.H., B.G.R., D.P., A.H. and A.P.

**Funding:** This work was supported by National Science Foundation COASTAL SEES #1325649 awarded A.P and D.P. K.L.P. was partially supported by student fellowships from the Geological Society of America (#: 11215-16), the Weigel Scholarship for Coastal Studies, Myers Oceanographic and Marine Trust and the Explorers Club Youth Foundation (#: 83402).

**Acknowledgments:** We would further like to acknowledge the students and researchers of the Paytan Lab at UCSC, in particular Joseph Murray, Ana Martinez Fernandez, Kim Bitterwolf, Kyle Broach and Katie Roberts and the scientific divers and boat crew, especially Jacque Lord and Rich Walsh, for their extensive help and support in completing this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
